The findings of this PhD provide a significant contribution to early intervention research. The ability to detect those at ultra-high risk for psychosis (UHR) has been made possible in recent years. It is well known that people with serious mental illness have poor physical health, yet prior to this PhD little was known about the physical health of UHR individuals. This PhD explores the physical health and lifestyle of the UHR group, and makes recommendations for the development of a physical health intervention. A range of methods have been used including quantitative and qualitative methods, systematic reviews and meta-analyses, and a clinical audit. Therefore, a multifaceted approach to investigate the physical health and lifestyle of UHR individuals has been taken. Papers 1-3 suggest UHR individuals are more likely to live an unhealthy lifestyle than their peers. This includes lower levels of physical activity, and higher levels of substance use (generally cannabis, tobacco and alcohol). Paper 4 contains a clinical audit showing physical health and lifestyle factors are not monitored routinely in early detection services, despite the UHR phase being an ideal opportunity to intervene. Living an unhealthy lifestyle can have a detrimental effect on physical and mental health. Papers 1-4 emphasise the need to intervene to promote a healthy lifestyle for the UHR group. In line with the Medical Research Guidelines for the development of complex interventions, a theoretical model is applied in Paper 5. The final paper presents a qualitative study with UHR individuals, their parents and clinicians to explore barriers and facilitators to living a healthy lifestyle and inform the development of a physical health intervention. A final evidence synthesis includes recommendations for future work and the clinical implications of this thesis. The findings of this PhD provide an important and timely contribution to early intervention research. Prior to this work, the physical health of UHR individuals had been largely under researched. For the first time, this PhD presents evidence to suggest individuals at ultra-high risk for psychosis experience cardiovascular risk, and there is an opportunity to intervene to promote physical health. Although not all UHR individuals will develop psychosis, many will continue to experience difficulties with their mental health. Given that this group are also more likely to live an unhealthy lifestyle, it is important to take a holistic approach to treating those at imminent risk for psychosis, considering both mental and physical health.