Engaging in sufficient physical activity (PA) and limiting sedentary time (ST) have major physical and mental health benefits Despite many previous theory-based interventions, the population prevalence of physical inactivity and ST remains high Three major shortcomings of previous behavior change interventions are (1) they do not consider the dynamic (time- and context-varying) nature of determinants, (2) they neglect the role of the physical environment and (3) effects on individuals (instead of groups) are underexplored This project has 2 main objectives to solve these shortcomings: (1) to examine the dynamic mechanisms underlying determinants and behaviors, (2) to test how behavior change can be initiated in individuals while considering the interaction with their environment The proposed theory-based model will be tested in older adults Two studies will be conducted A first observational study will use ecological momentary assessment to capture the context- and time-varying nature of individual-level and physical environmental determinants of PA and ST In a second study, the results of study 1 will be used to test isolated mechanisms expected to increase PA or decrease ST To do so, 4 micro-randomized trials (MRTs) will be conducted An MRT is a novel study design designed to solve dynamic questions about intervention components The ultimate goal of these MRTs is to find out which intervention is most effective for whom under which circumstances