Purpose: An objective database for physical education (PE) is important for policy and practice decisions, and the System for Observing Fitness Instruction Time (SOFIT) has been identified as an appropriate surveillance tool for PE across the nation. The purpose of this review was to assess peer-reviewed studies using SOFIT to study K-12 PE in U.S. schools. Method: The Preferred Reporting Items for Systematic Reviews and Meta-Analyses informed the review, and 10 databases were searched for English-language articles published through 2016. A total of 704 records identifying SOFIT were located, and 137 full texts were read. Two authors reviewed full-text articles, and a data extraction tool was developed to select studies and main topics for synthesis. Results: Twenty-nine studies that included direct observations of 12,256 PE lessons met inclusion criteria; 17 were conducted in elementary schools, 9 in secondary schools, and 3 in combined-level schools. Inconsistent reporting among studies was evident, including not all identifying the number of classes and teachers involved. All studies reported student physical activity, but fewer reported observer reliabilities (88%), lesson context (76%), teacher behavior (38%), and PE dosage (34%). The most frequently analyzed independent variables were teacher preparation (48%), lesson location (38%), and student gender (31%). Conclusions: SOFIT can be used reliably in diverse settings. Inconsistent reporting about study procedures and variables analyzed, however, limited comparisons among studies. Adherence to an established protocol and more consistent reporting would more fully enable the development of a viable database for PE in U.S. schools.