Fit for duty is a broad concept meaning an employee is physically capable, mentally alert, and free from substance impairment — able to safely perform their job's essential functions. While "fitness-for-duty evaluation" refers to the clinical examination, "fit for duty" describes the ongoing state employers need to verify. Fit-for-duty determinations may involve DOT physicals, drug and alcohol testing, functional capacity evaluations, or targeted medical examinations. The Americans with Disabilities Act (ADA) restricts when employers can require medical assessments — they must be job-related and consistent with business necessity. Fit-for-duty concerns are most critical in safety-sensitive roles where impairment could result in serious injury or death.