Background: The use of mobile phones for physical activity (PA) promotion and assessment represents an attractive research area because this technology is characterized by a widespread reach and dynamic features. Purpose: The purpose of this study was to conduct a systematic review of mobile phone-based approaches for encouraging and assessing PA. Methods: Eligible studies were identified through a search of PubMed and PsycINFO and required to address the usability of mobile phones for PA promotion and/or measurement, be written in English, and be published in a peer-reviewed journal between 2010 and 2013. Results: Fifty-two studies were included in this review. The use of mobile phones for PA promotion, assessment, and both promotion and assessment were addressed by 27, 19, and 6 studies, respectively. Out of 26 studies that reported a PA behavior change outcome, 13 observed favorable PA changes, 3 found mixed results, and 10 found no improvements. Ten studies examined the validity of mobile phones for PA assessment and 9 reported favorable outcomes. Participants found mobile phones to be highly acceptable. Discussion: The findings suggest that mobile phones have great potential as a PA promotion and assessment tool among a variety of individuals. Translation to Health Education Practice: Mobile phones may provide practitioners with efficient ways to promote PA and valid ways to assess PA.