The ARC Study is a national observational cohort study to investigate patient reported and clinical outcomes after hip and knee replacements. The study has the following objectives: 1. To evaluate outcomes following hip and knee replacements. 2. To determine risk factors for adverse outcomes in primary hip and knee replacements. 3. To identify targets for future research and intervention in patients undergoing primary hip and knee replacements 4. To provide a resource from which to identify potential participants for future clinical trials, and to use data collected in the ARC Study as comparison or control data for trial participants who have been randomised to receive one or more interventions. Participants are recruited and consented online. Patients are invited to participate by collaborating surgical teams nationally in outpatient clinics at the time of being added to a waiting list, in addition to an advertising campaign to recruit patients. They are directed to an online portal where they will be able to review further information. Consent and data collection is completed electronically through patients entering data online. Baseline demographics and characteristics are recorded, including details of socio-demographics, lifestyle, health status and patient reported outcome measures (PROMs). Patients then undergo hip/knee replacement and postoperative rehabilitation according to the standard care and protocols of the hospital and the preferences of their treating surgeon. Follow-up data, including PROMs, is collected via online questionnaires up to two years following surgery. The study will ultimately also enable multiple trials to be embedded within the cohort study, using a 'Trials within Cohorts' (TwiCs) methodology.