Guides for OpenAIRE Services

OpenCitations

Infrastructure dedicated to the publication of open bibliographic and citation data

What it is

OpenCitations is an independent not-for-profit infrastructure organization for open scholarship dedicated to the publication of open bibliographic and citation data by the use of Semantic Web technologies. OpenCitations espouses the UNESCO principles of Open Science and complies with the Principles of Open Scholarly Infrastructure, the FAIR data principles (data should be findable, accessible, interoperable, and reusable), and with the recommendations of I4OC that citation data, in particular, should be structured, separable, and open.
OpenCitations collaborates with a wide network of Open Science Infrastructures and organisations and is involved in communities focusing on the creation of a more equal and open research environment, including the Barcelona Declaration on Open Research Information, of which OpenCitations is among the supporting organisations.

What it does?

OpenCitations is dedicated to the publication of open bibliographic and citation data, thereby providing a disruptive alternative to traditional proprietary citation indexes.
OpenCitations provides

a data model: the OpenCitations Data Model (based on the SPAR Ontologies);
bibliographic and citation data (CC0): OpenCitations Index and OpenCitations Meta;
software: in our GitHub repository, released with open source licences;
online services: REST APIs, SPARQL endpoints, dumps and interfaces.

Bibliographic citation data have a fundamental value for the world of scholarship, and their open availability is a crucial requirement for the bibliometrics and scientometrics domains. OpenCitations increases the creation of reproducible metrics for research assessment since it provides open data usable for national and international research evaluation exercises. In particular, OpenCitations' main databases for bibliographic information are OpenCitations Index and OpenCitations Meta.

OpenCitations Meta is a database that stores and delivers bibliographic metadata for all publications involved in the OpenCitations Index. Every entity in OpenCitations Meta is assigned a persistent internal identifier called OpenCitations Meta Identifier (OMID);
OpenCitations Index is a collection recording citations between bibliographic resources. In Index, citations are treated as first-class data entities and are identified by an Open Citation Identifier (OCI).

OpenCitations ingests its information from multiple sources, each of which provides open citation data that has been merged and enriched in the unified collection OpenCitations Index.

OpenCitations Data Sources

Since 2022, the systematic integration of new data sources has necessitated a workflow reengineering to optimize execution time, storage space, and coding effort. The new workflow was launched at the end of 2023, with the aim of performing an efficient metadata crosswalk from the specific data models and file formats adopted by the data sources toward the OpenCitations data model (OCDM) and the formats we use as input to our subprocesses (CSV) to publish data (CSV, SCHOLIX, RDF).

The new ingestion workflow

As of July 2025, OpenCitations Index contains more than 2.2 billion citations derived from five reference sources. The citations are published under the CC0 public domain waiver and can be accessed via a flexible API. The following scheme shows the overlapping of the different sources, and the unique citations:

Data derived from the July 2025 data dump

OpenCitations enables:

Fairness, since it avoids institutions and independent scholars having to pay tens of thousands of dollars annually (that most of them cannot afford!) for commercial access to their own scholarly data;
Reuse, since there are no licence restrictions and all data, are provided under CC0, so users can re-publish and reuse for any purpose the citation data that OpenCitations provides;
Research assessment, since we provide open data usable for national and international research evaluation exercises, enabling one to make such activities transparent and reproducible;
Community governance, since the community is directly involved in the evolution of the infrastructure.

Using OpenCitations is beneficial because it:

Provides (without charge under open licences) open metadata with a scope, depth, accuracy and provenance, as a disruptive alternative to traditional proprietary citation indexes;
Supports stakeholder communities of libraries (authors, researchers, students, institutional administrators) to monitor research;
Enables reproducibility in bibliometrics research;
Tracks the scholarly productivity and influence of members of academic community;
Exploits free availability of citation data to build new applications and visualisations;
Guarantees easy access to the citation data through different points of access, depending on the particular need.

How can we use it?

One of OpenCitations’ main priorities is to keep its services, software and data always without charge under open licenses for fostering maximum reuse.
The data held in any of the OpenCitations datasets are made freely available under a Creative Commons public domain dedication (CC0).
The text of the web pages that comprise the OpenCitations web site is made freely available under a Creative Commons Attribution 4.0 International Public License.
The software developed by OpenCitations for implementing all the services is made freely available on GitHub under the ISC License.

OpenCitations coding material
https://github.com/opencitations

OpenCitations datasets search interface
https://search.opencitations.net/

OpenCitations query documentation
https://opencitations.net/querying/

OpenCitations data download
https://download.opencitations.net/

Technical Requirements

For using the services for programmatic access to the data (i.e. SPARQL endpoints and REST APIs), it is necessary to have a working internet connection and to specify the queries in a compliant way with the Web standards adopted (i.e. SPARQL and the HTTP protocol).
For downloading the dumps, it is necessary to have enough available space on the hard drive so as to correctly store and uncompress the archives – information about the size of the archives is available in the OpenCitations download page.