The measurement of clinical outcomes in trauma research is often problematic in that it is subjective and currently no feasible gold standard evaluation is available. Consequently, observed trial results are partly dependent on which outcome measure is used. Precise and useful estimates of treatment effects can only be obtained when using reliable, valid, and responsive instruments for measuring fracture healing. This overview outlines the concept of the validation of outcome measures and provides a summary of available and frequently used instruments in orthopaedic clinical trials. Outcome instruments can be divided into assessments by the clinician and assessments by the patient. Clinician-assessed measures are frequently used in routine practice but have often not been validated before their use in research. They include clinical and radiographic assessments. In contrast, patient-assessed measures have been designed specifically for investigational purposes and measure health on various domains. Some of them have been validated extensively. Critically evaluating established clinician-based assessments and integrating those found to be valid with patient-assessed outcomes into a composite measure of fracture healing constitute major future challenges.