The main goals of knee replacement surgery are pain relief and improvement of physical function as well as the long survivorship of the prosthesis. Traditionally, these outcomes were evaluated by physicians but meanwhile a variety of outcomes are of further interest and measurement shifted from the clinician’s perspective to that of the patient. Outcome measurement distinguishes between clinician-based outcomes (CBOs), patient-reported outcomes (PROs), patient-reported experience, and performance-based outcomes. Range of motion, joint stability, and radiographs are the standard CBOs before and after knee replacement. PROs in this context reflect on pain, walking ability, activities of daily living, physical activity, satisfaction, expectations and health-related quality of life. PROs are measured via generic or disease-specific instruments. When deciding on an outcome practical issues need to be considered. CBOs require direct patient contact, and PROs can be assessed by postal or online survey. Shorter questionnaires enhance the acceptance by the patients but reduce the possibilities for analysis. The used measurement instruments should fulfill a minimum of methodological quality to ensure comparability to other studies. The time points of measurement should be sufficient in answering the research question (e.g. survivorship of prosthesis at long-term follow-up).