The NCBO Resource Index Gem is a Ruby client for NCBO's Resource Index Web service. The NCBO Resource Index is a system for ontology based annotation and indexing of biomedical data; the key functionality of this system is to enable users to locate biomedical data resources related to particular concepts. A set of annotations is generated automatically and presented through integration with BioPortal, enabling researchers to search for biomedical resources associated (annotated) with specific ontology terms. This service uses a concept recognizer (developed by the National Center for Integrative Biomedical Informatics, University of Michigan) to produce a set of annotations and expand them using ontology is_a relations.