Mental disorders, including substance abuse, are part of the Mexican epidemiologic scenario and will remain so during several decades. They may even become more prominent as causes of disease, disability, and death in our country. It is thus imperative to frame appropriate management strategies to curb these problems without delay. This paper aims at outlining epidemiology of mental diseases as a field of study, and to identify its limitations. Emphasis is made on common elements shared with other more traditional fields of epidemiology, as well as on the specific contributions made by this particular field to epidemiology and to psychiatry in general. This paper describes the main study designs and problems in this field of epidemiology, its usefulness in prevention actions, and future challenges. A unique characteristic of mental disorder epidemiology is that its target diseases manifest in two levels: behaviorally (for example, compulsive hand-washing) and as an element of the individual's mental life (e.g., obsession with bacteria being a constant, omnipresent health threat). It follows that much of the knowledge currently available on the phenomena of mental disorders in general is based on the self-reported insight of individuals. Trained clinicians have collected such reports by interview or with standardized questionnaires. This field of epidemiology is characterized by having two-sides: a mental disorder is a problem in and of itself, causing suffering and prompting the search for specialized care, as it has peculiar clinical manifestations. On the other hand, mental disorder epidemiology also focuses on determining factors (drug use, abuse, or addiction) and on the way these independent variables result in certain processes and outcomes (such as accidents, homicide, suicide, liver cirrhosis, etc.). Finally, the epidemiology of mental disorders has also been set apart by its focus in series of processes that are not suitably classified as syndromes, but which are germane to public health, for example, violence. The epidemiology of mental disorders faces great challenges in the new millennium, including a complex, changing epidemiologic scenario. Several important issues will influence the future development of mental disorder epidemiology: measurement of mental disorders and risk factors, more efficient sampling design and methods, the relationships among biological research, genetics, social studies, and epidemiology, and the interface between epidemiology and the evaluation of therapies and health services.