OBJECTIVE: To quality assess a sample of health behavior change apps from the NHS Apps Library using a rating tool based on the 2014 National Institute for Health and Care Excellence behavior change guidance (NICE BCG). METHODS: A qualitative analysis of the NICE BCG identified themes and questions for a quality assessment of health behavior change apps. These were refined by further discussion and piloting, and applied by two independent raters to a sample of NHS Library apps (N=49). Disagreements were resolved following discussions with a third rater. RESULTS: Themes identified were; purpose, planning, usability, tailoring, behavior change technique (BCT), maintenance, evaluation, data security and documentation. Overall, purpose of the apps was clear, but evidence for collaboration with users or professionals was lacking. Usability information was poor and tailoring disappointing. Most used recognized BCTs but paid less attention to behavior maintenance than initiation. Information on app evaluation and documentation was sparse. CONCLUSIONS: This study furthers the work of the NHS Apps Library, adapting the NICE (2014) behavior change guidance for quality assessment of behavior change apps. PRACTICE IMPLICATIONS: This study helps lay the foundations for development of a quality assurance tool for mobile health apps aimed at health behavior change.