Fit to Fight Childhood Cancer

Inclusion Criteria: * Oncological disease according to the International Classification of Childhood Cancer. * Planned or started anti-cancer treatment (chemo- and/or radiotherapy and/or surgery) at the Clinic and Polyclinic for Paediatrics and Adolescent Medicine of the University Medical Centre of the Johannes Gutenberg University Mainz or connection to the Paediatric Oncology Centre Mainz within the framework of aftercare. * The patient is assessed by the treating team (pediatric oncologist, exercise professional etc.) as suitable to participate in the trial, e.g. due to medical or psychological reasons. * Existing informed consent (or assent) to participate in the study. * The patient is not in a terminal phase of the disease. Exclusion criteria: * The patient is assessed by the treating team (pediatric oncologist, exercise professional etc.) as unsuitable to participate in the trial, e.g. due to medical or psychological reasons. * After detailed information and, if necessary, having time to consider, the patient (≥ 16 years of age) did not agree to give written informed consent to participate in the trial. In the case of minor patients (\<16 years of age): The legal guardians do not assume that the child/adolescent is able to freely decide on participation or to consent to participation on the basis of the information received, and do not consent to this themselves. * The patient (and the legal guardians) has/have insufficient knowledge of the German or English language, so that it is not possible to carry out both the informed consent and interviews (in age-appropriate language). * The patient is in a terminal phase of the disease.