Background: In the Netherlands, inequalities in physical activity behaviour go hand in hand with socioeconomic inequalities in health. To stimulate physical activity behaviour and promote physical activity effectively and equitably, participatory community-based physical activity interventions seem promising. The Dutch government's policy is to support community-based sport and physical activity schemes at municipal level, on the assumption that participation in these programs supports the development of social capital, the quality of life in a community, and health and wellbeing. Although many strategies have been developed to increase physical activity levels in general and in socially vulnerable groups in particular, most evaluations show only small to moderate effects. To date, the evidence base rests mainly on correlational, cross-sectional studies at participant level, lacking insight into causal relationships and interaction patterns between factors influencing physical activity. In addition, in line with Dutch health promotion policy, there is a general demand for community-based health-enhancing physical activity (CBHEPA) programs to be evaluated for impacts and (cost) effectiveness. Aim: The aim of this thesis is to report on the design and implementation of an evaluation approach, assessing the effectiveness of CBHEPA programs at different impact levels (individual, group, and program), and the mechanisms involved. This study aims to contribute to the evidence base of programs targeting socially vulnerable groups, by applying systematically a multilevel and realist perspective in order to generate recommendations about how to evaluate physical activity promotion interventions targeting socioeconomic inequalities in health and physical activity. Methods: The study was built on a mixed methods design, combining quantitative techniques and qualitative approaches, to monitor 268 participants in 19 groups in seven ongoing CBHEPA programs between 2012 and 2015. We collected data at multiple levels. At individual level, a sequential cohort design was used to acquire quantitative longitudinal data on developments in physical activity behaviour and health-related indicators, and to assess participants' willingness to pay for sport and physical activity. At group and program level, interviews and focus group qualitative techniques of measurement were used. Thus, we were able to link outcomes at multiple impact levels from different datasets over a period of time, adding contextual and time-related value to our findings. The different kinds of evidence pulled from all cases contributed to the robustness of the mixed methods approach and to the generalisability of the findings. Results: Part I of this thesis presents the theoretical orientations for the development of a context-sensitive monitoring and evaluation approach in order to measure the effectiveness of CBHEPA programs. It presents an evaluation design, grounded in an ecological perspective on human health, enabling the identification of underlying mechanisms at multiple levels which explain what works and why in community-based physical activity programs. Part II presents the empirical findings from multiple perspectives. A multilevel analysis highlights the longitudinal developments from a participant perspective, addressing (leisure-time) physical activity behaviour in relation to participants' personal factors and covariates. CBHEPA programs reach socially vulnerable, but not necessarily inactive, groups in terms of socioeconomic and health-related quality of life outcomes. No increase in physical activity levels over time was observed, but the findings suggest that ongoing CBHEPA programs in particular contribute to physical activity maintenance in socially vulnerable groups. Over time, significant positive associations were found between leisure-time physical activity, and health-related quality of life, self-efficacy, and enjoyment. Furthermore, participants' willingness to pay (WTP) for sports and physical activity was explored - as also its associated predictors - in terms of money and time.