Abstract: Approximately 15,000 people leave the UK Armed Forces each year, with most having a positive transition to civilian life. However, a minority of ex-Service personnel experience challenges during transition, including unemployment, poor mental health, homelessness and involvement with the criminal justice system. These challenges can impact Service leavers’ and their families’ wellbeing in the short and long term, but may also present significant costs to government and the third sector. As part of broader research on the contemporary transition process and experiences thereof among UK Service leavers and their families, this report examines the cost of poor military-to-civilian transition. We examined the prevalence of adverse transition-related outcomes among Service leavers, the cost of these outcomes to the public and charity sector and the estimated total annual cost of adverse outcomes experienced by individuals in the four years after they leave the Armed Forces. While limited data on the prevalence of some adverse transition-related outcomes is a barrier for more robust cost estimates, we discuss the estimated cost of poor transitions and the potential of interventions and programmes within the sector that could reduce this cost alongside improving Service leaver outcomes.