This package consists of Perl modules along with supporting Perl programs that implement the semantic similarity and relatedness measures described by Leacock & Chodorow (1998), Wu & Palmer (1994), Nguyen and Al-Mubaid (2006), Rada, et. al. 1989, Jiang & Conrath (1997), Resnik (1995), Lin (1998), Banerjee and Pedersen(2002), Patwardhan and Pedersen (2006) and a simple path based measure. UMLS::Similarity requires the UMLS::Interface module to access the Unified Medical Language System (UMLS) in order to determine the similarity between two UMLS concepts. The Perl modules are designed as objects with methods that take as input two concepts from the UMLS. The semantic relatedness of these concepts is returned by these methods. A quantitative measure of the degree to which the two concepts are related has wide ranging applications in numerous areas, such as word sense disambiguation, information retrieval, etc. For example, in order to determine which sense of a given word is being used in a particular context, the sense having the highest relatedness with its context word senses is most likely to be the sense being used. Similarly, in information retrieval, retrieving documents containing highly related concepts are more likely to have higher precision and recall values.