Comprehensive evaluation of prior interventions designed to increase preschoolers' physical activity is lacking. This systematic review aimed to examine the effect of interventions on objectively measured physical activity in children aged 2-5 years. We followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement. In May 2014, we searched PubMed, CINAHL, PsycINFO, ERIC, SPORTDiscus, Cochrane, and Embase. Two reviewers independently identified and appraised the studies. Twenty-four articles describing 23 independent studies and 20 unique interventions met inclusion criteria. Of the 8 interventions resulting in a significant effect in objectively measured physical activity, all were center-based and included a structured physical activity component, 6 included multiple components, 5 integrated theories or models, and 4 actively involved parents. Seven of the 8 were randomized controlled trials. Due to the heterogeneity of the study designs, physical activity measures, and interventions, drawing definitive conclusions was difficult. Although the overall intervention effect was less than optimal, the review indicated that theory-driven, multicomponent interventions including a structured physical activity component and targeting both parents and their children may be a promising approach for increasing preschoolers' physical activity and warrant continued investigation using rigorous designs to identify those that are most effective.