Growing demands on the transportation system and constraints on public resources have led to calls for more private sector involvement in the provision of highway and transit infrastructure through what are known as "public-private partnerships" (PPPs). A PPP, broadly defined, is any arrangement whereby the private sector assumes more responsibility than is traditional for infrastructure planning, financing, design, construction, operation, and maintenance. This book describes the wide variety of public-private partnerships in highways and transit, but focuses on the two types of highway PPPs that are generating the most debate: the leasing by the public sector to the private sector of existing infrastructure and the building, leasing, and owning of new infrastructure by private entities.