Assessment of fracture union is a critical concept in clinical orthopaedics; however, there is no established "gold standard" for fracture healing. This review provides an overview of the problems related to the assessment of fracture healing, examines currently available tools to determine union, discusses the role of functional outcomes in the assessment of fracture healing, and finally evaluates healing outcome measures as they pertain to fracture trials. Because there is no universally accepted method to determine fracture healing, orthopaedic surgeons must rely on a range of tools that can include: radiographic assessment, mechanical assessment, serologic markers, and clinical evaluation (including functional outcomes). When used in conjunction, these tools can help to improve the sensitivity and specificity of determining fracture union. This is furthermore relevant when conducting fracture healing trials, for which there is little consensus between surgeons or the Food and Drug Administration as to optimal study endpoints. Such studies should therefore include a composite outcome measure consisting of radiographic and functional assessments to increase the quality and consistency of fracture healing trials.