Transportation is generally concerned with the efficient, safe, and sustainable movement of people and goods. Transportation engineers work on various aspects of the five stages essential in the life cycle of a transportation facility: planning, designing, building, operating, and maintaining. In the planning stage, we typically forecast traffic demands for a future year/analysis period, perform a preliminary evaluation of alternative solutions, or identify priorities for system improvements. This text provides a comprehensive and concise treatment of the topic of traffic flow theory and includes several topics relevant to todays highway transportation system. It provides the fundamental principles of traffic flow theory as well as applications of those principles for evaluating specific types of facilities (freeways, intersections, etc.). Newer concepts of Intelligent transportation systems (ITS) and their potential impact on traffic flow are discussed. State-of-the-art in traffic flow research and microscopic traffic analysis and traffic simulation have significantly advanced and are also discussed in this text. This textbook is meant for use in advanced undergraduate/graduate level courses in traffic flow theory with prerequisites including two semesters of calculus, statistics, and an introductory course in transportation. The text would also be of interest to transportation professionals as a refresher in traffic flow theory, or as a reference.