No standard criteria exist for diagnosing fracture nonunion, and studies suggest that assessment of fracture healing varies among orthopaedic surgeons. This variability can be problematic in both clinical and orthopaedic trauma research settings. An understanding of risk factors for nonunion and of diagnostic tests used to assess fracture healing can facilitate a systematic approach to evaluation and management. Risk factors for nonunion include medical comorbidities, age, and the characteristics of the injury. The method of fracture management also influences healing. Comprehensive evaluation includes an assessment of the patient's symptoms, signs, and immune and endocrine status as well as the biologic capacity of the fracture, presence of infection, and quality of reduction and fixation. Diagnostic tests include plain radiography, CT, ultrasonography, fluoroscopy, bone scan, MRI, and several laboratory tests, including assays for bone turnover markers in the peripheral circulation. A systematic approach to evaluating fracture union can help surgeons determine the timing and nature of interventions.