A general overview is provided of two primary categories of objective monitors: pedometers and accelerometers. The desired outcome measure will determine the specific instrument category, options, and features from which the ultimate instrument choice is made. Other considerations include evidence of validity and reliability, cost, computer interface and download options, memory capacity, data aggregation and storage methods, and general participant and researcher burden associated with instrument use. There is no single objective PA assessment instrument that is appropriate for all situations, populations, and research questions.