Primary care differs considerably from specialist mental health settings: problems are presented in undifferentiated forms, with consequent difficulties in distinguishing between distress and disorder, and a complex relationship between psychological, mental and social problems and their temporal variations. Existing psychiatric diagnostic systems, including ICD-10-PHC and DSM-IV-PC, are often difficult to apply in primary care. They do not adequately address co-morbidity, the substantial prevalence of sub-threshold disorders or problems with cross-cultural applications. Their focus on diagnosis may be too restrictive, with a need to consider severity and impairment separately. ICPC-2, a classification system created specifically for use in primary care, provides advantages in that it allows for simple linkage between reason for encounter, diagnosis and intervention. It is both necessary and feasible to develop a classification system for mental health in primary care that can meet four basic criteria: (1) characterized by simplicity; (2) addressing not only diagnosis but also severity, chronicity and disability; (3) feasible for routine data gathering in primary care as well as for training; and (4) enabling efficient communication between primary and specialty mental health care.