Background In the UK, the number of people with dementia is increasing along with life expectancy. Over half of the £23bn annual cost of dementia is due to informal care time. Therefore, there is an economic argument for identifying clinically-effective and cost-effective ways to maintain/ improve carer quality of life (QoL). This thesis explores the suitability of a capability based instrument, the ICECAP-O, for measuring QoL in informal carers of people with dementia. Methods Systematic reviews, qualitative interviews, survey data and clinical trial data were used. Construct validity of the ICECAP-O was examined using baseline data from the Challenge FamCare study of people with dementia and challenging behaviours, and an online survey created to collect additional data. A cost-effectiveness analysis was undertaken using a subsample of carers in the REMCARE trial of joint-reminiscence therapy (RT). Results The current evidence base for interventions to support carers of people with dementia is poor, with few economic evaluations existing. Qualitative work found the major themes affecting carer QoL overlapped well with capability instruments, suggesting this framework is suitable for carer research. Validity work found the ICECAP-O to be feasible and valid for use with carers. At the end of the 10 month RT trial, the mean difference in ICECAP-O scores between groups was -0.02 (bootstrapped 95% CI of -0.105 to 0.066) and the mean difference in costs was £1,464 (bootstrapped 95% CI of £758 to £2,313), RT was dominated by usual care. The cost-effectiveness acceptability curve indicated a 2% probability that RT was cost-effective at a willingness to pay threshold of £20,000 per point improvement and an 8% probability that RT was cost-effective at a threshold of £30,000 per point improvement on the ICECAP-O. Conclusion To date, the ICECAP-O has not been used with carers of people with dementia; this thesis offers a unique insight into its suitability from theoretical validity through to applied research. When assessing the cost-effectiveness of interventions targeting outcomes other than physical functioning, I encourage researchers to include capability based instruments in an attempt to bridge the gap between measuring physical functioning and measuring a more encompassing quality of life.