Psychiatric illnesses are some of the most common and important comorbid conditions seen in neurologic practice, and they also represent important contributors to disability in many patients. Moreover, neuropsychiatric symptoms such as depression and anxiety are increasingly recognized as prodromal or even core symptoms in neurologic disorders such as Parkinson’s disease, Alzheimer’s disease, and multiple sclerosis. This chapter explores how psychiatric disorders are defined and diagnosed, as well as many aspects of managing and treating them.