YouCan! is a therapy-accompanying app for adolescents and young adults with cancer for the prevention of therapy-related side effects and secondary diseases. The app offers those affected scientifically based psychological support, with target group-specific functionalities to promote active disease management and increase compliance, thus saving time for all involved and costs for clinics and health insurance companies. With the help of documentation of one's own course of treatment, daily mood and symptom queries, exchange with other affected persons, and age-specific psychoeducational content, YouCan! promotes active coping with the ups and downs of the disease and thus helps those affected to gain new courage.