Depression is a common psychiatric comorbidity in the major neurologic disorders (e.g, stroke, epilepsy, migraine, Alzheimer’s dementia, multiple sclerosis, and Parkinson’s disease), with average prevalence rates of 25% to 40%. The relation between depression and several of these neurologic disorders is bidirectional, that is not only are patients with these neurologic conditions at greater risk of developing depression, but patients with depression are at greater risk of developing these neurologic disorders. Furthermore, the presence of comorbid depression has been associated with a worse course of the neurologic disorder and a higher risk of failure to respond to the neurologic therapies. This chapter reviews the epidemiologic and clinical characteristics of depression in the major neurologic disorder and describes the impact it has on the course of the neurologic condition and response to treatment. Finally, it identifies those neurologic disorders in with a bidirectional relation has been identified and suggests potential pathogenic mechanisms that may be operant in their complex relation.