This paper presents an overview of the elements of traffic assignment and a synthesis of the problems that may be encountered in applying traffic-assignment models in practice. The elements include preparing the transportation network, establishing the origin-destination (OD) demands, identifying a traffic-assignment technique, calibrating and validating a model, and forecasting. The practical problems of traffic assignment are classified into four categories: network-representation problems, system-subarea data-translation problems, model-calibration problems, and forecasting problems. The paper presents the state of the art in traffic-assignment elements and provides specific guidelines on their implementation in practice and on the resolution of the identified problems. This paper is one of a series being produced by the Urban Transportation Division (UTD) Transportation Planning Committee of ASCE.