AbstractLittle is known about behavioral transfer and compensation within and between moderate-to-vigorous physical activity (MVPA) and sedentary behavior. Thus, taking a within-person perspective, this study investigated across 1 week whether (a) children and adolescents compensate for increased MVPA and sedentary behavior with less of the respective behavior the next day and (b) transfer and compensation occur between these behaviors within 1 day and across 2 days. We obtained data from 2676 participants (6–17 years) of the national Motorik-Modul (MoMo) study in Germany. Participants wore an ActiGraph accelerometer (Pensacola, FL, USA) for 7 days. We analyzed within- and between-day associations using hierarchical linear modeling. If youth engaged in 2 h more sedentary behavior than typical on any given day, they engaged in 37.20 min less MVPA the same day (B = −0.31, p < 0.001) as well as in 4.80 min more MVPA (B = 0.04, p < 0.001) and 7.20 min less sedentary behavior (B = −0.06, p < 0.001) the next day. If youth engaged in 1 h more MVPA than typical on any given day, they engaged in 97.80 min less sedentary behavior the same day (B = −1.63, p < 0.001) and in 5.40 min less sedentary behavior the next day (B = −0.09, p < 0.001). No association with next-day MVPA was observed. Our results indicate that children do not compensate for enhanced MVPA but transfer to less sedentary behavior, while more sedentary behavior is compensated with less sedentary behavior and more MVPA the next day. This provides essential information for the design of intervention studies to tackle physical inactivity and sedentary behavior.