The ability to correctly diagnose spinal non-union is vital to our ability to diagnose and treat patients with new or recurrent symptoms following spine fusion and to accurately assess the efficacy of spine fusion techniques and technologies. Surgical exploration has traditionally been the gold-standard investigation for spinal non-union. As routine surgical exploration is impractical in the majority of patients the use of non-invasive radiologic methods of spine fusion assessment is necessary. The purpose of this paper is to outline the most common radiologic methods of spine fusion assessment including the strengths and limitations associated with each imaging modality. In addition we will review the best-available evidence for the use of radiologic investigations to diagnose spine non-unions. We will then provide recommendations for what we believe to be the best methods of diagnosing successful union of cervical interbody, lumbar interbody and lumbar posterolateral fusions that can be used by spine clinicians and researchers alike.