Within 10 years of experiencing a deep vein thrombosis of the lower limb, up to 60% of people will be classed as suffering from post-thrombotic syndrome (PTS). The cause and risk factors for PTS are not well understood and there are no universally agreed diagnostic criteria. This thesis aimed to identify prognostic factors associated with developing PTS, the method(s) of diagnosing PTS and their relative reliability in identifying PTS. A systematic review of systematic reviews and a systematic review of primary studies was conducted to identify prognostic factors. Methods used to diagnose PTS were noted from these reviews. Prognostic factors from best evidence and methods of diagnosing PTS noted were presented to clinical experts for prioritisation via an e-Delphi study. Consensus was defined as ≥75% agreement. Fifty one potential prognostic factors and seventeen methods of diagnosing PTS were identified from the reviews and initial exploration of experts’ views. Experts reached consensus on eight prognostic factors and one method of diagnosing PTS. The prognostic factors identified can be considered for the development of a prognostic model, while the method of diagnosing PTS found to be most reliable from experts’ opinion should be considered when developing a reference standard for PTS diagnosis.