Monitoring is a self-regulatory process involved in making changes to behavior. Monitoring involves a person, group, or organization taking stock of the current situation, comparing this to some goal or reference value, and identifying whether or not there is a discrepancy. Noting a discrepancy can be a reason for taking additional action to ensure goals are achieved or for adjusting or disengaging from the goal. Monitoring can also identify actions required to overcome barriers to goal striving and whether these actions have the intended effects. Given that people often do not monitor their progress, termed “the ostrich problem,” interventions that prompt monitoring can be an effective way to promote changes in behavior. This chapter reviews the evidence that monitoring interventions promote changes in behavior, identifies how monitoring has been conceptualized within theoretical models and existing taxonomies of behavior change techniques, and describes some of the mechanisms by which monitoring promotes behavior change. The chapter concludes that monitoring can be an effective strategy for promoting changes in a range of behaviors and contexts but also that developing monitoring interventions can be complex. A practical guide for the development and application of monitoring strategies is also presented, based on the literature and research evidence on monitoring interventions.