This document summarizes the NCBO Resource Index, which is an ontology-based search tool that indexes over 16 billion annotations from over 200 ontologies and 3.5 million elements. It integrates information from diverse biomedical data sources and ontologies to improve search results through synonyms, hierarchies and mappings. The Resource Index is developed by the National Center for Biomedical Ontology to help users search for, find and explore biomedical resources.