Self-change is a well-documented phenomenon in the addiction field. Because many individuals with alcohol problems do not seek treatment due to stigma, cost, and their problems are not severe, developing interventions that can be implemented outside of clinical settings is important. In this regard, while electronic/online interventions have become popular, evidence-based interventions for mobile phones are scarce. Mobile phone applications (apps) have portability and can be accessed and used by a wide and diverse range of people. Despite these advantages, evidence-based apps to treat alcohol problems do not exist. The need to reach the "unreachable" resulted in the Guided Self-Change (GSC) treatment model being evaluated as a mail intervention in two randomized clinical trials (RCTs) in Canada and the US. The mail intervention consisting of personalized feedback materials was very cost-effective, reached large numbers of individuals who had never sought treatment before, and helped these individuals reduce their drinking. The advent of phone apps provides an opportunity to reach an even greater number of individuals who otherwise may not seek services. This study will develop and evaluate an iPhone app based on the GSC mail intervention, after which the app will be tested for usability. The app will include smoking self-monitoring logs because research shows that heavy drinkers are more likely to be smokers, and because a major factor associated with drinking relapse is smoking. A second phase will involve a RCT using a two-group design comparing the app with its evidence-based counterpart, the mail intervention, using a 3-month follow-up with participants recruited via advertisements. The clinical and public health implications of this project are considerable. Extending the evidence-based mail intervention to an evidence-based app would provide privacy, immediate personalized feedback about alcohol and tobacco use and its risks, and most importantly, 24-hour access to a therapy-based intervention.