Health care policy, systems, and investment in related research have increasingly advocated for the personalisation of routine treatment decisions in order to improve population health outcomes. Stratified medicine may facilitate such personalisation by targeting specific health technologies to subgroups of patients defined by their individual-level characteristics. The management of patients with chronic diseases, in particular, such as rheumatoid arthritis, may be revolutionised by stratified medicine if treatments are subsequently prescribed conditional on patient-level heterogeneity. Economic evidence is essential to demonstrate that stratified medicine is a cost-effective use of finite health care resources to support its expansion within the National Health Service in England. The aim of this thesis was to provide evidence for the relative cost-effectiveness of a biomarker test to stratify treatment for patients with rheumatoid arthritis, consistent with the requirements of decision-makers for the National Health Service in England. A specific case study of stratified medicine was selected (adalimumab anti-drug antibody and drug level testing) that was early in its product lifecycle and had an emerging, but limited, supporting evidence base. The thesis addressed three specific research questions: (i) What was the existing economic evidence for stratified medicine in rheumatoid arthritis? (ii) How were treatment decisions with biologic therapies made for patients with rheumatoid arthritis in current practice in England? (iii) Are treatment decisions stratified by adalimumab anti-drug antibody and drug level testing, for patients with rheumatoid arthritis in England, a relatively cost-effective use of health care resources? The research questions were answered by using a mixed methods approach (systematic reviews; qualitative thematic framework analysis; quantitative econometric analysis; decision analytic modelling by discrete event simulation). The individual studies within the thesis built towards the overall aim sequentially and were linked together by a common theme of generating relevant evidence to inform decision-making for a new stratified medicine in England. A de novo early model-based economic evaluation of adalimumab anti-drug antibody and drug level testing for patients with rheumatoid arthritis in England was conducted, informed by (i) the limitations of published economic evaluations of stratified medicine for rheumatoid arthritis, (ii) a substantial characterisation of the care pathways in current practice for patients with rheumatoid arthritis in England, and (iii) an extensive process to conceptualise the structure of the decision analytic model. Stratified medicine by adalimumab anti-drug antibody and drug level testing for patients with rheumatoid arthritis in England was found not likely to be a relatively cost-effective use of health care resources based on current evidence. There was considerable decision uncertainty associated with this result and further prospective research, in particular on test accuracy and health outcomes, were found to be of substantial value to the health care system. This thesis made clear contributions to knowledge by advancing the economic evidence base for stratified medicine in rheumatoid arthritis and by demonstrating the value and challenges of generating economic evidence early within the product lifecycle of a new stratified medicine.