Pollution, accidents and congestion all cause unnecessary welfare losses, and while transport services are an essential component of economic and social development, their negative side effects are drawing increasing political attention. Internalisation aims to create incentives to reduce these external costs by factoring them into markets. This report summarises the theoretical and practical dimensions to internalisation; reviews recent estimates of external costs; explores the mix of policies that might be used to promote internalisation successfully; and estimates the size of incentives required in monetary terms.