Mental health, mental illness and stress-related disability are especially ill-defined, complex and controversial issues when considered in the context of the workplace. Currently, a knowledge gap exists between mental health professionals and employers regarding symptom-based models of illness and function based models of work performance. As a result, psychiatric disorders affecting workers are under-identified and under-treated and likely result in unmitigated impairment and disability. The authors examine several conceptual models for workplace mental illness across medical and psychological disciplines and propose a unifying construct. The utility of the existing screening methods for common workplace illnesses and their potential application are reviewed. The challenges of diagnosis and effective treatment of workplace mental illness are highlighted within an “occupational mental health system” with suggestions for future research directions.