We are fortunate to have several nosologies that recognize insomnia as an independent or primary disorder. The various classification systems provide us the wherewithal to differentiate types of insomnia by both the presenting complaint and the factors that are thought to precipitate or perpetuate the illness. Perhaps what remains to be accomplished, from a definitional point of view, is for scholars and scientists to complete the characterization of this important disorder by providing for the formulation of a definition which formally lays out the research diagnostic criteria for insomnia. While this may not be essential for (in general) clinical practice or (in specific) for the determination of when CBT-I is indicated, a formal and complete definition will allow clinical researchers to conduct more productive investigations when the entity under study may be evaluated in its “chemically pure” form.