Both depression and conduct disorders are relatively prevalent and are related to poor long-term outcomes. Despite being characterized by very different symptoms, it is well established that these two disorders co-occur at higher rates than expected by chance, resulting in poorer adjustment for the individual than would result from either problem alone. The termcomorbidityis usually reserved to refer to the association of diagnosed disorders, whereasco-occurrencerefers more broadly to the association of levels of symptoms of conduct problems and depression, which are usually calculated with means or possibly symptoms counts. In the past two decades, researchers have focused particularly on the following issues regarding the comorbidity of depression and conduct disorder: (1) possible causal associations of the two problem behaviors (i.e., do depressive disorders tend to onset after conduct disorders or vice versa); (2) theory regarding causes of the association (i.e., common versus unique risk factors for these two problem behaviors); (3) changes across development (i.e., with age); (4) risks from diagnosed disorders versus symptoms that do not reach diagnostic criteria; (5) outcomes or prognosis (e.g., are outcomes more severe for co-occurring problems than for either problem alone, are there distinct patterns of outcomes associated with co-occurring problems). Within each of these areas there is considerable interest in moderation of effects by gender or gender similarities and differences. This chapter reviews findings pertaining to these issues and presents suggestions for future research. In addition, assessment approaches and clinical implications are discussed.