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BASE, operated by Bielefeld University Library (Germany), is one of the world's most voluminous search engines especially for academic open access web resources. It provides more than 50 million documents (about 75% available in full text) from more than 2,700 sources. BASE (Bielefeld Academic Search Engine) is a multi-disciplinary search engine to scholarly internet resources, created by Bielefeld University Library in Bielefeld, Germany. It is based on free and open-source software such as Apache Solr and VuFind. It harvests OAI metadata from institutional repositories and other academic digital libraries that implement the Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH), and then normalizes and indexes the data for searching. In addition to OAI metadata, the library indexes selected web sites and local data collections, all of which can be searched via a single search interface.

Users can search bibliographic metadata including abstracts, if available. However, BASE does not currently offer full text search. It contrasts with commercial search engines in multiple ways, including in the types and kinds of resources it searches and the information it offers about the results it finds. Results can be narrowed down using drill down menus (faceted search). Bibliographic data is provided in several formats, and the results may be sorted by multiple fields, such as by author or year of publication.

Paying customers include EBSCO Information Services who integrated BASE into their EBSCO Discovery Service (EDS). Non-commercial services can integrate BASE search for free using an API. BASE becomes an increasingly important component of open access initiatives concerned with enhancing the visibility of their digital archive collections.