In the physical activity study, study participants in the intervention group receive an interactive, computerized expert system (Intervention Group, IG). Individuals in the Active Control Group (ACG) get a standard program, which does not tailor treatment components to the individual needs of the patients. Internet users (N = 1000) will be recruited in in the internet via press releases, personal invitations and advertised likes at well known web-sites. Study participants will be followed up over three measurement points: One to eight weeks after T1, T2 will take place. Four to 12 weeks after T1 the T3 measurement will be conducted.