Although considerable progress has been made in understanding hypersomnia, the pathophysiology of idiopathic hypersomnia (IH) is still totally unknown. This article details the concept of IH with long sleep time and without long sleep time, and reports the main differential diagnosis. Also explored is hypersomnia caused by medical conditions, hypersomnia following infection, and nonorganic hypersomnia. There is a definite need further to develop sleep laboratory investigations to assess the correct diagnosis. Studies at the genetic and biologic levels are also needed to further the understanding of the pathophysiology of IH and to develop specific treatment.