"Thesauri are valuable structures for Information Retrieval systems. A thesaurus provides a precise and controlled vocabulary which serves to coordinate document indexing and document retrieval. In both indexing and retrieval, a thesaurus may be used to select the most appropriate terms. Additionally, the thesaurus can assist the searcher in reformulating search strategies if required. This chapter first examines the important features of thesauri. This should allow the reader to differentiate between thesauri. Next, a brief overview of the manual thesaurus construction process is given. Two major approaches for automatic thesaurus construction have been selected for detailed examination. The first is on thesaurus construction from collections of documents, and the second, on thesaurus construction by merging existing thesauri. These two methods were selected since they rely on statistical techniques alone and are also significantly different from each other."