The author first reviews the technology history of EM as well as the factors that led to its use in criminal justice contexts. This is followed by an overview of the methodologies and findings of EM evaluations. Despite the common assertion that EM is primarily used with offenders who would otherwise be imprisoned, evidence indicates that there has been "net-widening" to include low-risk offenders who would not normally be incarcerated. Thus, the low recidivism rate attributed to some EM programs merely reflects the low-risk profile of the participants ("Martha Stewart" effect). EM is typically used as a monitoring tool that carries mild restrictions in a scheme of graduated sanctions; however, its effectiveness is often compromised by technical problems and uncertainty about how to address monitoring violations by offenders. Few, if any, programs have used EM primarily or exclusively as a positive reinforcement tool. This occurs when EM is used to measure performance for the purpose of providing rewards for positive behaviors. EM can track attendance at treatment programs, job sites, and other rehabilitative activities. EM-measured attendance and promptness can then be rewarded by various means to reinforce positive behaviors. This paper argues that EM and other mobile technologies should be used primarily to reinforce prosocial behavior rather than as a sanction.