Editing Citation index (section)

==Major citation indexing services==
{{main article|Indexing and abstracting service}}
{{main cat|Citation indices}}
General-purpose, subscription-based academic citation indexes include:
*[[Web of Science]] by [[Clarivate Analytics]] (previously the Intellectual Property and Science business of [[Thomson Reuters]])<ref>{{cite web |url=https://clarivate.com/webofsciencegroup/solutions/web-of-science/ |title=Web of Science |website=Clarivate |access-date=April 24, 2022}}</ref>
*[[Scopus]] by [[Elsevier]], available online only, which similarly combines subject searching with citation browsing and tracking in the sciences and [[social sciences]].
Each of these offer an index of citations between publications and a mechanism to establish which documents cite which other documents. They are not open-access and differ widely in cost: Web of Science and Scopus are available by subscription (generally to libraries).

[[CiteSeer]] and [[Google Scholar]] are freely available online. 

Several open-access, subject-specific citation indexing services also exist, such as:
*[[INSPIRE-HEP]] which covers high energy physics,   
*[[PubMed]], which covers life sciences and biomedical topics, and 
*[[Astrophysics Data System]] which covers astronomy and physics.