As the volume and specialisation of research continue to grow, making findings immediately accessible helps researchers build on each other’s work more quickly, fostering collaboration and speeding up innovation.

Open Access (OA) makes research openly available at no cost to the reader, facilitating the widespread dissemination of essential research and further scientific discovery.

At a glance, the evolution of OA can be traced across four immensely influential declarations: The Budapest Open Access Initiative, the Bethesda Statement on Open Access Publishing, the Berlin Declaration on Open Access to Knowledge in the Sciences and Humanities, and, most recently, Plan S.

In this article, we explore these four essential declarations in the history of OA, uncovering how each built upon the previous one to contribute to OA as we know it today.

The Budapest Open Access Initiative

Published in February 2002, the Budapest Open Access Initiative (BOAI) is the landmark and arguably most fundamental declaration in the history of OA. It not only marks the first use of ‘Open Access’ as an official term but provides a clear mission statement for what OA is and the core principles that support it.

At this time, a handful of scholarly platforms such as MDPI’s first journal Molecules and arXiv were already championing OA values by making research easily available online. However, there was not yet a shared term to unite these efforts. Therefore, BOAI represents a defining shift towards a coordinated movement that had, until then, operated on the periphery of scholarly publishing.

More than just coining the term itself, the BOAI set out to define exactly what OA publishing was and ‘brought together leaders who were exploring alternative publishing models’ to explore how scientific research could be made more widely accessible. The BOAI defined OA as:

“By ‘open access’ to this literature, we mean its free availability on the public internet, permitting any users to read, download, copy, distribute, print, search, or link to the full texts of these articles, crawl them for indexing, pass them as data to software, or use them for any other lawful purpose, without financial, legal, or technical barriers other than those inseparable from gaining access to the internet itself.”

Alongside this definition, the BOAI proposed two potential avenues in which scholars could achieve the widespread and free accessibility of their research. This included self-archiving in repositories, known as Green OA, or publishing in OA journals, known as Gold OA. A testament to the enduring influence of the BOAI, both remain central to OA publishing today.

A blueprint for a bold idea

The Budapest Open Access Initiative (BOAI) was a radical and aspirational declaration which provided an exciting alternative to the traditional subscription-based model which restricts essential research behind expensive paywalls.

Importantly, the Initiative left key gaps for future initiatives to address. So, while it introduced the concept of OA in broad, ideological terms, it allowed further discussion regarding the technical requirements that an OA publication should adhere to.

In this sense, the BOAI was critical for establishing what OA should stand for, rather than specifying exactly how it should be implemented.

Bethesda Statement on Open Access Publishing

Taking place in the following year (2003), the Bethesda Statement on Open Access Publishing aimed to stimulate discussion within the biomedical research community on how to rapidly and efficiently implement Open Access (OA).

Where BOAI provided an essential foundational ideology and definition of OA, the Bethesda Statement continued discussions by elaborating on what exactly qualifies as an OA publication.

At the Howard Hughes Medical Institute in Maryland, USA, the Bethesda Statement was drafted during a meeting between scientists, librarians, publishers, and funders of the biomedical research community. They set out to “agree on significant, concrete steps” which should be taken to accelerate the transition to OA.

This resulted in the recommendation of two important specifications that constitute an OA publication:

• Firstly, the attendees concluded that an OA publication must first permit “free, irrevocable, worldwide, perpetual right of access to… copy, use, distribute, transmit, and display the work publicly“.
• Secondly, such works must be immediately deposited in a publicly accessible repository.

This was significant as it established a more precise, testable definition of Open Access than what had been put forward previously. It outlined key technical requirements, rather than just what ‘openness’ meant within this context.

A definition without institutional backing

This meeting established clearer parameters for OA in practice. However, despite the statement making recommendations to key actors in the academic ecosystem, still nothing obliged these researchers, institutions, publishers, or funders to adopt OA practices.

Additionally, a key caveat of the Bethesda Statement is the idea that the attendees, despite holding senior roles across various institutions, “participated as individuals”. This meant that while these actors held clear influence in their field, endorsement of OA practices had not yet reached an institutional or governmental level.

Berlin Declaration on Open Access

In many ways, the Bethesda Statement provided a solid test for what qualified as an Open Access (OA) publication. Yet, as the Statement highlighted, this endorsement remained at an individual level.

Published in the same year, the Berlin Declaration on Open Access to Knowledge in the Sciences and Humanities set out to close that gap by providing an evolution of existing ideas supported by the weight of institutional signatories.

The Berlin Declaration originated from a conference hosted by the Max Planck Society and the European Cultural Heritage Online Project in Berlin, Germany, and was written “in accordance with the spirit of the Declaration of the Budapest Open Access Initiative, the ECHO Charter and the Bethesda Statement on Open Access Publishing”.

It expanded discussions by broadening its scope across all academic disciplines and defined OA as “a comprehensive source of human knowledge and cultural heritage that has been approved by the scientific community”.

The Berlin Declaration was signed by various institutional leaders. This included the presidents of German research institutions and organisations, alongside other international bodies such as CNRS in France. By 2005, 55 institutions had signed the Declaration, including national scientific academies across countries such as China, India, and the Netherlands.

As of August 2026, this number has surpassed 800 global institutions.

This was a key development from the previous declarations. These signatures were explicitly provided ‘on behalf’ of the research organisations they represented and therefore demonstrated a growing commitment to advancing OA on a larger scale. This gave the movement, for the first time, a formal institutional mandate rather than an individual one.

The BBB definition of Open Access

Together, the Budapest, Bethesda, and Berlin Declarations have fundamentally shaped the Open Access movement (OA). They provided solid foundations for OA and its overarching goal of delivering the free availability and unrestricted reuse of scholarly works. For this reason, they are often collectively referred to as the BBB definition of OA.

Still, clear principles, technical requirements, and institutional endorsement did not directly ensure widespread adoption; nothing across these declarations forced research to be published in an OA format.

Closing that final gap came in the form of widespread funder mandates, which had direct financial leverage over researchers themselves.

Plan S

In 2018, the European Commission supported a group of national research funding organisations in launching cOAlition S, which aimed at accelerating the transition to OA. This endeavour was built around ten core principles known as Plan S.

From 2021, Plan S mandated “full and immediate Open Access to peer-reviewed scholarly publications from research funded by public and private grants”. In this case, satisfying ‘full and immediate’ OA requires research to be published under an open licence that permits full reuse, notably CC-BY, with no embargo period.

Explicitly informed by the previous declarations, Plan S’s first principle directly cites the Berlin Declaration. Importantly, the key difference is enforcement. Rather than simply asking institutions to support OA, funders now mandate it and some grant holders enforce sanctions for researchers who fail to comply.

OA mandates or policies are rules or requirements set by governments, funding agencies, or universities that require work to be published in an OA format. Plan S is just one of a growing list of international OA mandates.

See our article ‘Open Access Policies and Mandates Around the World’ to learn more.

Open Access is the new paradigm

By exploring these declarations in tandem, we can understand how they directly built on one another, rather than serving as separate, overlapping statements of the same idea.

In summary, the declarations evolved as follows:

• Budapest defined the goal of OA but left its terms undefined.
• Bethesda provided those terms without introducing an obligation to adopt them.
• Berlin introduced institutional adoption at scale but stopped short of enforcement.
• Plan S strengthened the enforcement 18 years later.

While these 4 examples are not a comprehensive account of all the milestones which have influenced Open Access, when explored together they uncover how the current position of OA as a widely accepted form of scientific publishing has been shaped through sustained cycles of refinement and evolution.

MDPI pioneers Open Access

MDPI has pioneered Open Access since 1996 and defines Open Access in accordance with the definitions provided by the Budapest, Berlin, and Bethesda Declarations.

All research published across MDPI’s portfolio of more than 500 Open Access journals is immediately available online worldwide and may be reused without permission, provided that it is clearly cited.

We are dedicated to giving you all the information you need to understand Open Access. Our article All You Need to Know About Open Access covers a range of topics.