Muscle weakness is a common complaint among patients presenting to family physicians. Diagnosis begins with a patient history distinguishing weakness from fatigue or asthenia, separate conditions with different etiologies that can coexist with, or be confused for, weakness. The pattern and severity of weakness, associated symptoms, medication use, and family history help the physician determine whether the cause of a patient's weakness is infectious, neurologic, endocrine, inflammatory, rheumatologic, genetic, metabolic, electrolyte-induced, or drug-induced. In the physical examination, the physician should objectively document the patient's loss of strength, conduct a neurologic survey, and search for patterns of weakness and extramuscular involvement. If a specific cause of weakness is suspected, the appropriate laboratory or radiologic studies should be performed. Otherwise, electromyography is indicated to confirm the presence of a myopathy or to evaluate for a neuropathy or a disease of the neuromuscular junction. If the diagnosis remains unclear, the examiner should pursue a tiered progression of laboratory studies. Physicians should begin with blood chemistries and a thyroid-stimulating hormone assay to evaluate for electrolyte and endocrine causes, then progress to creatine kinase level, erythrocyte sedimentation rate, and antinuclear antibody assays to evaluate for rheumatologic, inflammatory, genetic, and metabolic causes. Finally, many myopathies require a biopsy for diagnosis. Pathologic evaluation of the muscle tissue specimen focuses on histologic, histochemical, electron microscopic, biochemical, and genetic analyses; advances in technique have made a definitive diagnosis possible for many myopathies.