 The objective of this study is to determine the differential effect of intervention delivery channel Tailored Internet versus Tailored Print on physical activity adoption and maintenance in previously sedentary adults. To our knowledge, no studies have examined the efficacy of a Tailored Internet-based physical activity intervention. Thus, we will conduct a randomized controlled clinical trial comparing three groups, 1 Internet-based motivationally-tailored individualized feedback Tailored Internet; 2 print-based motivationally-tailored individualized feedback which has previously shown to be efficacious Tailored Print; and 3 websites currently available to the public Standard Internet comparison group. Two hundred and forty healthy, sedentary women and men ages 18-65 will be randomly assigned to one of the three groups. The Tailored Internet and Tailored Print arms will be matched on frequency and content of contact. Data will be collected at baseline, 6, and 12 months using well-established physical activity and physical performance measures, as well as a comprehensive set of psychosocial questionnaires. Our primary hypothesis is that individuals randomized to the Tailored Internet arm will exhibit significantly higher levels of physical activity participation at 6 and 12 months than individuals in the Tailored Print arm due to the enhanced capabilities of the Internet e.g., immediacy of feedback and enhanced graphics. And, in addition, subjects randomized to the Tailored Internet or Tailored Print conditions will exhibit significantly higher levels of physical activity participation at 6 and 12 months than those in the Standard Internet comparison condition. Other questions of interest will include an examination of potential moderators and mediators of the intervention-physical activity relationship. This study will contribute important information regarding the relative efficacy of Internet and print-based interventions, each of which can be utilized for widespread public health dissemination. 