This chapter examines the emergence and continued growth of the motor carrier industry as the core of the US freight transportation system. The development of the US motor carrier industry during the twentieth century resulted from a combination of historical circumstance as well as timing of technological innovations, government intervention, and economic development that led to changes in freight markets and the demand for trucking services. The chapter reviews the structure of the motor carrier industry is presented to provide enough background and terminology necessary to understand its evolution over time. It considers key factors influencing industry development in each era as well as policy issues that have emerged as both the motor carrier industry and the overall economy have changed. One of the most important distinctions to make in any discussion of the industry is the difference between truckload (TL) and less-than-truckload (LTL) trucking operations.