Experience has shown that manual data collection via ridechecks is one of the most cost-effective data collection methods for most transit systems when evaluating ridership levels and system performance. Unfortunately, this manual method of collecting ridership data and system operational information produces both limited systemwide and route specific reports due to the amount of manual ridechecking required and the manual data processing involved. This is especially true for larger transit systems. One possible alternate solution for a transit system to meet its need for reliable, accurate, detailed, up-to-date, and cost-effectively-obtained ridership data is through the use of automatic passenger counters, or APCs. Unlike manual ridechecks, an APC system provides a transit system with an automated method for collecting information on the number of passenger boardings and alightings at a variety of system levels including route, route segment, or specific bus stops by time of day and by day of week, for example. This paper provides a general overview of APC systems. Included is a discussion of APC components, a review of typical transit system data needs, a list of current worldwide APC vendors, the results of a survey of North American transit systems related to APC usage, and a summarization of important issues for transit systems to consider when contemplating integrating APC systems on board their vehicles.