Physical activity is important for health; however, people’s lifestyle is becoming more sedentary and inactive (Straker, Coenen, Dunstan, Gilson, & Healy, 2016). Prolonged sitting and physical inactivity cause serious health issues, such as Non-Communicable Diseases and obesity (Grace & Dunstan, 2018). In Australia, 70% of adults are sedentary or insufficiently active (ABS, 2013). Although adverse health effects increase if a person sits for more than seven hours, sedentary workers are under the threat of this issue for sitting eight to 10 hours per day at work (ABS, 2013). Alternatively, breaking up sitting by light physical activity is recommended (Stuart, 2016). However, solving this problem is not straight forward because of the complex nature of the behaviours (Chastin et al., 2016). Social marketing is a potential approach to tackle such a complex issue. Social marketing uses marketing concepts to influence behaviours for individuals and society (Lefebvre, 2013), which previously achieved behaviour change including physical activity (e.g. Fujihira et al., 2015; Kubacki et al., 2015). However, theory use in social marketing remains scant (e.g., Truong & Dang, 2017). Furthermore, previous social marketing approaches have been predominately individual-focused even though causal factors may also exist in the surrounding environment. Therefore, there is a need for more innovative formative research, together with theory, to identify the leverage points in the system from a broader perspective (Kubacki & Rundle-Thiele, 2013). Study 1, Systematic literature review was conducted to identify how physical activity interventions targeting working-age adults used five sources of self-efficacy between 2007 and 2017. Following PRISMA guidelines, this study reviewed 19 qualified papers across 13 studies. The results found which self-efficacy techniques are associated with changes in self-efficacy for physical activity interventions. Mastery experience was the most commonly used technique in this review and highlights the need to offer more flexible options for participants and the inclusion of self-monitoring as part of the strategies to achieve behaviour change. Overall, results were mixed across studies highlighting the importance of specifying exactly which self-efficacy source-based techniques are used in intervention studies, and ensuring techniques are delivered as intended (with fidelity) and engaged with (Walton et al., 2017). Future research should also examine how differences in cursorily similar techniques have differing amounts of effect on selfefficacy and behaviour. Study 2, focus group held two sessions with sedentary office workers to obtain consumers’ insights. Guided by the socio-ecological model, the result confirmed that barriers and facilitators to reducing occupational sitting exist across multiple ecological levels, particularly at the organisational level. The participants’ voices indicated that an individual's psychological state and behaviour can be influenced by social and environmental settings, including self-efficacy. The results further highlighted the potential importance of self-efficacy as a psychological facilitator in physical activity and sedentary behaviour interventions. Sources of self-efficacy are modifiable and exist across multiple levels. Therefore, the findings reveal that a more nuanced understanding of self-efficacy to increase light physical activity is important to reverse the current negative trend. Study 3, online survey was conducted to explore light physical activity at work from two different theoretical perspectives namely self-efficacy theory and social cognitive theory. Based on the findings from study 1 and 2, study 3 aimed to examine the validity among sources of self-efficacy and social factors and compare two theories to identify which theory is more appropriate to examine light physical activity at workplaces. The result demonstrated discriminant validity among five sources of self-efficacy and social factors. Furthermore, self-efficacy theory was found to hold stronger explanatory power than social cognitive theory. Although social cognitive theory is one of the most commonly used theories in physical activity and sedentary reduction contexts, this finding showed the need for more in-depth research on the sources of selfefficacy. Overall, this doctoral research contributed to the literature theoretically and practically. First, this research contributed to the social marketing literature by providing an overall understanding obtained from research that used multiple theories (self-efficacy theory, social cognitive theory, and socio-ecological model). The combination of these three theories enabled the author to examine the context from a different theoretical angle without losing the focus on the key factor namely self-efficacy. Aligning with the recent call among social marketing scholars, this research showed the importance of taking a broader perspective to understand the context. However, this research also highlighted the importance of in-depth research on the individual level construct such as self-efficacy.