Insomnia is a prevalent complaint that may arise from myriad causes. Therefore, patients who present for insomnia evaluation and treatment represent a rather heterogeneous group that merits a reliable and valid diagnostic system. This review article considers the general purposes of diagnostic classification per se and highlights the factors that influence the development of diagnostic nosologies. Various past and current insomnia nosologies are described, and data supporting the reliability, validity, and general utility of these systems are presented. In addition, the limitations of existing nosologies are discussed, and factors that will lead to improved insomnia nosologies in the future are considered. The panel discussion that follows this review highlights the limitations of current nosologies and notes barriers that must be overcome to improve upon the currently available classification systems.