There has been a rapid recent increase in both the number and type of objective physical activity (PA) assessment instruments which are commercially available to researchers, practitioners, and consumers. Although this has provided improved capacity for PA assessment, it also presents a somewhat bewildering range of options related to instrument selection for users of these technologies. The purpose of this review is to provide a primer to guide selection of instruments for the objective monitoring of children's PA. In an effort to inform without overwhelming, it is not intended to be exhaustive in terms of all available instruments. A general overview is provided of two primary categories of objective monitors: pedometers and accelerometers. Within each category we focus on distinctly relevant options and features important to consider during instrument selection. In general, 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. Further, we can anticipate that the commercial nature of these instruments will drive an even greater range of features and options in the future, increasing both the opportunity and the challenge for objectively assessing PA in children.