The diagnostic concepts of insomnia are controversial. New diagnostic criteria of insomnia disorder have been published in the International Classification of Sleep Disorders (ICSD-3) and the Diagnostic and Statistical Manual of Mental Disorder (DSM-5). Both the ICSD-3 and DSM-5 did not accept the concept of secondary and primary insomnia. According to the ICSD-3 and DSM- 5, insomnia disorder can be diagnosed, even with a comorbid mental or sleep disorder. The medical, psychiatric, and sleep history are essential for the diagnosis of insomnia disorder. Polysomnography can be helpful for differential diagnoses or the clinical subtyping of insomnia disorder. Recently, the European and American Sleep Society published clinical guidelines for the treatment of insomnia. Both guidelines strongly recommended non-pharmacological treatment (e.g., cognitive behavioral disorder for insomnia) as a treatment of choice for insomnia. Pharmacological treatments for insomnia are weakly recommended only for a short time. New non-pharmacological and pharmacological treatments for insomnia disorder are being developed and studied.