Originally described for the treatment of tuberculosis of the spine, spinal fusion has become a widely done procedure for various spinal disorders. Among other indications, spinal fusion is used for treatment of degenerative disc disease, congenital and developmental deformities such as scoliosis, and spinal instability secondary to trauma. One of the most common complications of spinal fusion is nonunion. The incidence of nonunion has been reported as high as 56% in the lumbar spine. Recognizing and treating nonunion of the spine may be crucial in preventing progressive deformity in identifying instrumentation failure, and in relieving persistent pain. The purpose of this article was to review the clinical evaluation and significance of failed union.