This paper describes the current research efforts into automatic vehicle identification being undertaken as part of the Heavy Vehicle Electronic License Plate (HELP) program. Automatic Vehicle Identification (AVI) is the term used for techniques which uniquely identify vehicles as they pass specific points on the highway, without requiring any action by the driver or by an observer. It constitutes one of the three key technological elements, along with weigh in motion and automatic vehicle classification, which combine to form the HELP system. There are a number of alternative AVI technologies and systems under consideration for use in the HELP System, which are each briefly described in the paper. A comprehensive test program is now under way to evaluate the performance of each of these systems and technologies, with the aim of producing a definitive specification for an AVI system which will meet the needs of the HELP system. This program is described in detail and future developments in AVI are discussed.