Background: High levels of sedentary behaviour – waking activities that involve sitting or reclining and a low amount of energy expenditure – are associated with negative health outcomes. University students are a population subgroup that is at risk of engaging in excessive sedentary behaviour, as a significant proportion of their time is spent studying or in class. The main aim of this PhD project was to lay the groundwork for and develop an intervention aimed at reducing and breaking up sedentary behaviour in university students using the Behaviour Change Wheel, a theory-driven intervention development framework (phase one). A second aim was to conduct a pilot trial on the feasibility and preliminary short-term results of the sedentary behaviour change intervention (phase two). Methods: The intervention development phase included systematic literature reviews and one-on-one interviews with university students (n = 18) to understand what needs to change for the target behaviour to occur, according to the Capability, Opportunity, Motivation, Behaviour model and the complementary Theoretical Domains Framework. Phase two consisted of piloting the behaviour change intervention using a quasi-experimental study design (n = 9). The intervention content was delivered through a face-to-face session, together with daily text messages reinforcing the key intervention messages (sit less – move more, more often). Outcomes were assessed at two different time points (pre/post) and included accelerometerbased (activPAL) and selfreported (NightlyWeekU) total sedentary time, as well as accelerometer-assessed number of steps and prolonged sedentary time. In addition to the outcome evaluation, a mixed-methods process evaluation informed by the UK Medical Research Council’s framework was included as part of the trial to assess the acceptability of implementation structures, clarify causal mechanisms, and identify relevant contextual factors. Results: Findings from phase one, together with previous literature, helped identify the ...