Health Technology Assessment (HTA) faces challenges such as the absence of direct treatment comparisons to evaluate technologies since health technologies get more complex. There is a growing interest that real-world data (RWD) could help fill evidence gaps in appraising new drugs, particularly cancer drugs. This thesis explored the use of RWD in appraisals of oncology medicines by the National Institute for Health and Care Excellence (NICE) to investigate how RWD have been used, how the use varies by cancer areas and over time, what factors are associated with greater or lesser use of RWD and to what extent RWD can help reduce uncertainty in the economic evaluation of cancer medicines. In this thesis, data were systematically extracted from 229 NICE Single Technology Appraisals of cancer drugs following a specially-developed data extraction protocol. Beyond simple counting, patterns and intensity of use of RWD were identified to review the use of RWD in appraisals. Patterns were then categorised based on the number of uses of RWD in three major components (overall survival (OS), volume of treatment, choice of comparators) for an analysis of the intensity of use of RWD. Seven factors (time, internal/external validity, availability of direct treatment comparison, incidence rate, maturity of the data on OS, previous technology appraisal recommendation by NICE) were identified, which could be possible incentives for greater use of RWD. Regression models were estimated to find the associations between the use of RWD and a set of the factors. An association between the use of RWD in estimating OS and the maturity of OS data was reviewed in depth. By highlighting the challenges, such as uncertain generalisability of a clinical trial and immature survival data, sources of uncertainty in appraisals and patterns of use of RWD were investigated. This was reviewed by comparing appraisals of targeted cancer therapy with appraisals of non-targeted cancer therapy. Lastly, appraisals of the cancer drugs exiting the 2016 Cancer Drugs Fund (CDF) were reviewed to understand how RWD on provision through the CDF helped address uncertainties in the review process. The research showed that NICE has incorporated RWD substantially in diverse parts of the economic evaluation. However, the use of RWD to reduce uncertainty in appraisals appears to be quite limited.