ABSTRACT: 
This paper describes grey literature, its origins, definitions and the role of grey literature in modern digital age and scholarly communication. 
Grey literature plays an important role in the rapid and timely distribution of in-depth, recent, scientific and technical information, and also provides access to a broad range of data and often contains new ideas. 
Furthermore, grey literature is an important source of original research and up to date information. 
In this paper we will analyze the role of 2014 Pisa Declaration on Policy Development for Grey Literature Resources and its contribution to better understanding and recognizing the role of grey literature as an essential resource in scholarly communication, research, policy making and a key source of evidence, argument, innovation, and understanding in many disciplines. 
The last part of this paper will investigate the relations between grey literature and scientific heritage, by introducing a set of criteria for the scientific heritage collections in order to define them as a grey literature.  
KEYWORDS: Grey literature, Pisa Declaration, GreyNet, scientific heritage  
The term ‘grey literature’ first appeared in scientific publications in the 1970s. 
Prior to this period, this term was associated with different types of technical, scientific and economic reports. 
At the same time, grey literature was often described as ‘informal,’ ‘non-conventional,’ ‘running away,’ ‘invisible’ or ‘half-published’ (Nahotko, 2008).  
Nowadays, grey literature is usually defined as a field in library and information science that deals with the production, distribution, and access to multiple document types produced on all levels of government, academics, business, and organizations in electronic and print formats, but not controlled by commercial publishing, i.e. where publishing is not the primary activity of the producing body (Sch¨opfel, 2010). 
Examples of grey literature include patents, technical reports from government agencies or scientific research groups, working papers from research groups or committees, white papers, and preprints.  
Grey literature may be made available to the public, or distributed pri-vately within an organization or group, and often lacks systematic means of distribution and collection. 
The standards of quality, review and production can also vary considerably. 
Grey literature is therefore often diffcult to discover, access, and evaluate. 
Among the most frequently enumerated features we can find the following common characteristics of grey literature resources: 
– diffcult to identify, access and locate; 
– often come in the form of limited editions; 
– inaccessible in bookstores; 
– lack of bibliography registration; 
– absent in library collections and catalogues and in a publisher’s catalogues as well; 
– diffcult to acquire in libraries; 
– tend to be unpublished or published with delay.  
All these characteristics have high impact on the inaccessibility of grey literature resources. 
The lack of bibliographic standards and regulations make them even more invisible and often hidden from the wide public.  
According to experts in the field, grey literature has as many definitions as there are forms of publications. 
This is mostly because of the ephemeral and changing nature of grey publication types, editions, and formats, as well as to the relative “newness” of the field. 
In addition, the evolving impact of the Internet since the mid-1990s has only “further muddied the definitional waters” (Rucinski, 2015). 
The metaphor “the chameleon of information resources” maybe describes the grey literature in best way, as “it can constitute of virtually anything and be written for and by anyone in almost any format” (Rucinski, 2015). 
The ephemeral and variable nature of grey publication types, editions, and formats makes it hard to describe and define. 
Despite that, there are many definitions of grey literature. 
The Prague definition from The Twelfth GL International Conference (Prague, 2010), defines grey literature as follows:  
Grey literature stands for manifold document types produced on all levels of government, academics, business and industry in print and electronic formats that are protected by intellectual property rights, of suffcient quality to be collected and preserved by library holdings or institutional repositories, but not controlled by commercial publishers i.e., where publishing is not the primary activity of the producing body (Sch¨opfel, 2010).  
Most of papers define grey literature as a set of various document types, above all reports, working papers and conference proceedings. 
Below are given some examples of different types and forms of grey literature: 
– Technical reports, working papers and proceedings (Japzon and Anderson, 2004); 
– Working papers, reports, newsletters, conference proceedings, committee reports, theses and dissertations, and government sponsored research reports (Costello, 2007); 
– Scientific reports (Stock et al., 2006); 
– Unpublished works, materials from conferences that are not readily avail-able and rare government reports (Gheen and Olmsted, 2010); 
– Conference papers, memos, technical drawings, project reports, the-ses (Biagioni and Giannini, 2009); 
– Government publishing, research reports of various kinds, guidelines for carrying out research, reprints, informational material from political parties, religious congregations, and associations (Juliusdottir, 2014); 
– Handwritten notes as grey literature sources (Cirković, 2016).  
Furthermore, these authors are mentioning also other characteristics such as: 
– Usually diffcult to discover or obtain, often diffcult to find in libraries, online databases and on the web, non-accessible, at risk of being lost without investment (Jackson, 2005); 
– Non-commercially published, not widely published or unpublished, “neither bibliographically accessible in catalogues open to the public nor available through traditional distribution channels of market publishing” (Juliusdottir, 2014).  
Ramos-Lum and Vogel (2005) cite the GreyNet Luxemburg definition of grey literature and distinguish it from “ephemera”, i.e. “materials (physical or electronic) that regardless of appearance, quality or quantity, at some point were considered disposable and of little or no value, through time, had become valuable in such a way that it had broadened their appeal and made them desirable to be collected and preserved by individuals, collectors and information institutions”. 
In fact, they describe “ephemera” as a special kind of grey literature.  
From 2006 to the present, the grey literature field has been in a state of tran-sition characterized by definitional changes, preservation challenges, and new cooperative research initiatives among various groups. 
These collaborations are embodied by projects as OpenGrey and the GreyGuide, a newly launched open resource project between GreyNet International and ISTICNR (Pisa, Italy), focused on the sharing and development of good practices in the field of grey literature. 
Today, the grey literature community continues to explore and challenge conventional norms through its international conference series and through its publications in the Grey Journal, which represents the only currently published scholarly journal dedicated to the field of grey literature.  
The Grey Literature Network Service (GreyNet)1 was founded in 1992 as the first scholarly association dedicated to the study of grey literature. 
Its mission is to facilitate dialog, research, and communication between persons and organisations in the field of grey literature. 
GreyNet further seeks to identify and distribute information on and about grey literature in networked environments. 
Its main activities include the International Conference Series on Grey Literature, the creation and maintenance of web-based resources, a moderated Listserv, a combined Distribution List, The Grey Journal (TGJ), as well as curriculum development in the field of grey literature. 
In 1993, GreyNet held its first International Conference on grey literature in Amsterdam, with subsequent global conferences held every two years to the present. 
The upcoming Twentieth International Conference on Grey Literature “Research Data Fuels and Sustains Grey Literature” will take place in New Orleans, Louisiana USA, on December 3-4, 2018. 
GreyNet is fully open access compliant. 
Authors and researchers in grey literature communities worldwide know that their metadata, full-texts, slide presentations, research data, and other outputs are preserved and made openly accessible to the broader public, available in the following repositories: 
– GreyGuide Portal and Repository; 
– OpenGrey Repository; 
– Dans Data Archive; 
– OpenAIRE European Repository Network; 
– WorldWideScience.org Global Science Gateway.  
In the time of fast information exchange, grey literature has an important role and offers to scientists faster and free access to the knowledge. 
There-fore, it is important to include grey literature in open access databases and repositories in order to increase recognition of grey literature for open access to research, open science and knowledge transfer.  
The Pisa Declaration on Grey Literature3 was developed at a forum held in Pisa, Italy in May 2014 organized by GreyNet and the National Na-tional Research Council of Italy. 
It is currently translated into 20 languages worldwide (English, Armenian, Bulgarian, Croatian, Czech, Dutch, French, German, Greek, Hindi, Hungarian, Italian, Japanese, Korean, Macedonian, Russian, Serbian Cyrillic, Serbian Latin, Spanish, Tagalog, Turkish).  
The Pisa Declaration provides a 15-point roadmap that should serve as a guide for organizations involved with the production, publication, access and use of grey literature well into this 21th century. 
Until now, the problem was the lack of cooperation and coordination between and among organizations dealing with grey literature. 
The Pisa Declaration marks an end to ad hoc policy and decision making with regard to grey literature resources. 
The main points set out in the Pisa Declaration can be grouped into five categories:  
1. Organizational commitment to open access and the sharing of open data standards;  
2. Commitment to research and education, where recognition and reward is associated with quality of grey literature, and where attention is given to good practices in the field;  
3. Commitment to address and safeguard legal issues inherent to grey literature by exploring the various types of licensing agreements now available and by fostering constructive relations with commercial publishers;  
4. Commitment to sustainability linked to a financial prerequisite. 
Identifying funding and grants for special collections and repositories, commitment to long-term preservation, and investments in new technologies;  
5. Firm technical commitment to continued online services and further cross-linking of textual and non-textual content – a commitment that ranges from tackling broken links to facilitating interoperability regardless of the system or portal in which grey literature and its accompanying data are housed (Farace, 2014). 
“It is in this way that the Pisa Declaration can revel in the strengths and opportunities that grey literature offers, while at the same time exposing the weaknesses and threats facing our community. 
No longer are we resigned that grey literature is hard to find, but instead how can we best search and access it. 
No longer hold in question its worth and value, but instead set out the review process it has undergone. 
And, no longer hesitate as to whether it is published or not, but instead cite and reference grey literature – make it openly public isn’t that what published means?” (Farace, 2014) 
There is no doubt that grey literature represents important resource of scientific research, and the main purpose of Pisa Declaration is to increase the awareness of scientific community, governments, organisations and all important bodies about the true value of grey literature and its usability and accessibility. 
One of the values and research potentials, which grey literature has, is its relation to a scientific heritage, which will be discussed in the following chapter.  
Why is it important to invest in older grey literature? 
Because these docu-ments are “hidden treasures” of historical value (Stock et al., 2006; Biagioni and Giannini, 2009), interesting for institutional history and commemoration (Anderson et al., 2007). 
In other words, for the authors who are writing about the older grey literature, the value is record of progress, not information itself. 
The value of older materials can be increased by making them more readily available: “The biggest challenge left to our library is to make our grey literature and ephemera collection available to our users” (Ramos-Lum and Vogel, 2005). 
This means a systematic search and collection of resources from a variety of agencies and organizations (Costello, 2007) or digitization and web dissemination of collections that were before fairly inaccessible (Gheen and Olmsted, 2010). 
Another related purpose is digital preservation which, along with dissemination through the different web platforms, prevents grey literature “from moving further toward black” (Ramos-Lum and Vogel, 2005). 
Another way of preservation would be through central depositories, especially institutional repositories (Jackson, 2005; Lynch, 2017).  
However, there are a lot of challenges which need to be overcome, such as, lack of bibliographic control, heterogeneity, locations in different places and organizations, in order to use grey literature in its full capacity. 
If we define grey sources as not published, it means we can consider handwritten notes as grey literature source in terms of scientific heritage – they are important for the scientific research, but they are not visible. 
That makes them grey, their non-visibility and non-obligation to be a part of bibliographic records. 
The way of increasing their visibility and usability, would be through introducing new bibliographic standards with special fields for grey literature sources, which will bring the new value in terms of valorisation of all different types grey literature.  
If the scientific heritage source is not published, and is not part of library catalogues, but is important in terms of scientific research, it could be considered as grey literature source.  
Grey literature is an important source for scientific research, since it provides rapid exchange and dissemination of information among the scientific community. 
However, it is still on margins and its importance is not recognized by wider community. 
There are a lot challenges which need to be overcome in order to give the grey literature the deserved place in academic community and scholarly communication.  
In last decade some significant steps have been made in order to increase the visibility, usability and wider public awareness about the grey literature. 
One of important events was publishing of Pisa Declaration on Policy Development for Grey Literature Resources in 2014, which provides us with clear guidelines how to treat grey literature sources in order to increase the awareness of scientific community, governments, organisations and all important bodies about the true value of grey literature. 
Still, there are many open questions, such as, older grey literature sources, their definition and determination with regard to scientific heritage, grey literature and open access sources, which are waiting to be answered and investigated in some future researches. 
