Although liver and kidney transplantation allowed for tremendous improvement in overall recipient survival, cardiovascular disease has emerged as a leading cause of death in this population. Poor physical fitness, only recovering slowly and incompletely after transplantation, adds to the heightened cardiovascular risk of hypertension, diabetes, dyslipidemia and obesity in these patients. The gut microbiome has emerged has a modulator of muscular, metabolic, and cardiovascular health. Liver and kidney transplantation are associated with microbial dysbiosis. Exercise-induced modulation of the gut microbiome could act as a mechanism for its health benefits. The gut microbiome could also be a predictor of the training response. We hypothesize that 6 months of remotely-mediated home-based exercise training succeeded by a physical activity intervention tailored to the patients’preferences is safe, cost-efficient, and effective in improving physical fitness, cardiovascular health, and the gut microbiome in de novo liver and kidney transplant recipients. The role of training intensity will also be evaluated in this multicentric RCT. Importantly, results will be easily and rapidly implementable in daily practice and result in sustained physical activity. 180 kidney and liver transplant recipients at UZ Leuven and UZGent will be randomly allocated to regular care (written advice on physical activity) or home-based exercise training. After an initial ~2-months training period at moderate intensity, patients are randomly allocated to moderate intensity or moderate- and high-intensity training for ~4 months followed by a physical activity program. Primary outcome is cardiorespiratory fitness assessed by peak oxygen consumption, a most vital clinical parameter. Secondary outcomes are cardiovascular health parameters, gut microbiome, safety (incl. infection incidence), costeffectiveness, and implementation potential.