The overall goal of this study is to test personalized mobile phone-based physical activity interventions among students at the University of California, Berkeley. Most physical fitness applications for smartphones and activity trackers use a constant goal for the number of steps each day. However, if the step goals are dynamically adjusted according to past behavior, then the corresponding goals may encourage individuals to increase their physical activity level. This study consists of a randomized controlled trial in which we are assessing the efficacy of four different algorithms for calculating personalized goals for the number of steps each day and for determining which motivational messages to send each day.

Inclusion Criteria:

• intent to become physically active
• own an iPhone 5s (or newer model)
• willing to keep the iPhone in pockets during the day
• willing to install and use the intervention app every day for 4 months
• ability to speak and read English.

Exclusion Criteria:

• known medical conditions or physical problems that require special attention in an exercise program
• planning an international trip during the next 4 months, which could interfere with daily server uploads of mobile phone data
• pregnant/gave birth during the past 6 months
• severe hearing or speech problem
• history of an eating disorder
• current substance abuse
• current participation in lifestyle modification programs or research studies that may confound study results
• history of bariatric surgery or plans for bariatric surgery in the next 12 months.