{Background Health behavior change theories usually claim to be universally and individually applicable. Most research has tested behavior change theories at the interindividual level and within young-to-middle-aged populations. However, associations at the interindividual level can differ substantially from associations at the intraindividual level. This study examines the applicability of the Health Action Process Approach (HAPA) at the inter- and the intraindividual level among older adults. Methods Two intensive longitudinal studies examined the HAPA model covering two different health behaviors and two different time spans: Study 1 (physical activity