Non-union is one of the most devastating complications after fracture fixation. It usually results in prolonged treatment duration and unpredictable results. We reviewed the literature to identify recent information regarding the following: risk factors of nonunion; mechanical risk factors, including fracture gap width and stability, osteonecrosis and healing mechanism, osteoporotic fracture and fixation method, the characteristics of fracture, soft tissue injury, local infection, and multiple fractures; as well as the metabolic risk factors, including age, comorbidities, smoking, alcoholism, and medications. The technique and devices for fracture treatment have been developed, and treatments of nonunion are evolving according to the enhancement of our understanding of nonunion. Clinicians should refer to the risk factors and advancements while developing a treatment plan.