With the emergence of mobile health (mHealth) apps, there is a growing demand for better tools for developing and evaluating mobile health interventions. Recently we developed the Personal Health Intervention Toolkit (PHIT), a software framework which eases app implementation and facilitates scientific evaluation. PHIT integrates self-report and physiological sensor instruments, evidence-based advisor logic, and self-help interventions such as meditation, health education, and cognitive behavior change. PHIT can be used to facilitate research, interventions for chronic diseases, risky behaviors, sleep, medication adherence, environmental monitoring, momentary data collection health screening, and clinical decision support. In a series of usability evaluations, participants reported an overall usability score of 4.5 on a 1-5 Likert scale and an 85 score on the System Usability Scale, indicating a high percentile rank of 95%.