Fitness-for-duty and return-to-work assessments represent core competencies within occupational medicine, requiring practitioners to integrate detailed knowledge of both job-specific demands and individual worker health status. This scholarly review examines the clinical, legal, and ethical dimensions of these evaluations, with particular emphasis on the Americans with Disabilities Act (ADA) and its international analogues. The author argues that fitness determinations must be understood as dynamic, context-dependent judgments rather than static medical certifications, necessitating ongoing reassessment as job roles or health conditions evolve. Key considerations include the three foundational criteria for evaluation—worker capacity, workplace risk, and reasonable accommodation feasibility—as well as the hierarchical categorization of outcomes (fit, fit with restrictions, or unfit). Special attention is given to emerging challenges, including mental health disabilities, post-viral functional impairments such as long COVID, and the persistent evidentiary gaps in validation of assessment tools. The review concludes that while fitness-for-duty evaluations are indispensable for workplace safety and legal compliance, they must be conducted with rigorous adherence to anti-discrimination principles, individualized assessment, and interdisciplinary collaboration to achieve optimal outcomes for both employees and employers.