This journal article describes the research protocol of the ongoing Log My Life study at the University of Southern California. This study uses a mixed-methods based on geographically explicit ecological momentary assessment (EMA) through cell phone technology to understand the risk environent of young adults who have either enrolled in housing programs or are currently homeless. The researchers collect data using web-based questionnaires and daily diaries through cell phones to understand the risk environments of young adults with regard to emotional affect, context, and health risk behavior, including infrequent risk behaviors such as sex in exchange for goods and services. They use EMA to look at how the study participants move around their environment throughout the day and whether these movements result in dangerous substance use and sexual activity. Mixing the quantitative and qualitative arms in this study will provide a more complete understanding of differences in risk environments between homeless and housed young adults. (author abstract modified)