An overwhelming amount of studies have demonstrated that exercise is beneficial for health. Exercise is medicine! But do people take the medicine? And for how long? How can exercise behaviour be increased? This thesis focusses on exercise behaviour which contains planned, structured, repetitive bodily movements with the intention to improve or maintain (physical) fitness or health. The trans theoretical model of behaviour change (TTM) is often used to understand changes in health related behaviour, such as exercise. The TTM is an integrative model, combining different theories and constructs. It describes four key variables that are the stages of change, decisional balance, self-efficacy and the processes of change. The applicability of this model to exercise behaviour of the 151 million members in fitness clubs worldwide has never been studied. The purpose of this thesis is to study exercise behaviour of this specific population, applying the framework of the TTM, to get more people, more active, more often!