The intention-behavior gap has proven to be a vexing problem for theorists and practitioners interested in physical activity. Intention stability is one factor which moderates this gap. This study articulated and tested contrasting views of intention stability as (a) a dynamic characteristic of people that influences assessment error (and therefore the predictive power of intentions) and (b) the product of a dynamic process that unfolds within people over time. Using an ecological momentary assessment design, young adults (N = 30) rated weekly physical activity intentions for 10 weeks and wore pedometers for the first 4 weeks of the study. Substantial within-person variability existed in intentions over both 4- and 10-week intervals, and this variability was not a function of time exclusively. Multilevel modeling revealed that overall intention strength (across weeks) and weekly deviations in intention strength interacted to predict weekday (but not weekend) physical activity. These findings indicate that the person and context interact to selectively couple or decouple intentions from daily physical activity.