Sleep is common to all animals and yet there are so many mysteries surrounding its function. Insomnia, the most common sleep disorder, is prevalent and debilitating and has been shown to play a role in the onset and maintenance of other mental health disorders, such as depression, anxiety, and bipolar disorder. This chapter outlines a basic overview of sleep, followed by a summary of cutting-edge research investigating insomnia as well as its relationship to other psychiatric disorders. It is suggested that a bidirectional relationship exists between sleep and emotion and research supporting this framework is addressed briefly. We describe the treatment of choice for insomnia, cognitive behavioral therapy, as well as outline other exciting developments in the treatment of insomnia, including bright light therapy, intensive sleep retraining therapy, and Internet-based treatments. Finally, we end on a discussion of areas that are ripe for future investigation, including biological mechanisms, sleep medications, and other sleep disorders such as hypersomnia.