Australia has faced the growing problem of overweight and obesity along with other developed countries and almost two thirds of Australians are considered to be overweight or obese (Sturgiss, van Weel, Ball, Jansen, & Douglas, 2017). The surrounding environment encouraging excessive food intake and discouraging physical activity is the main cause for the current overweight and obesity epidemic (Velema, Vyth, & Steenhuis, 2017). Studies show that Australian Military personnel are no exception, even though considered as physically fit, they possess poor dietary habits (Booth & Coad, 2001; Forbes-Ewan, Probert, Booth, & Coad, 2008; Skiller, Booth, Coad, & Forbes-Ewan, 2005) and the occurrence of obesity is similar when compared to the general Australian population (Australian Institute of Health and Welfare, 2010). There is a pressing need for effective programs that can accomplish positive behaviour change to decrease the incidence of overweight and obesity in the Australian Defence Force. Social marketing is a discipline that applies commercial marketing techniques and concepts to improve the welfare of individuals and/or society by centring program design on individuals and an understanding of the environment in which they are located (Gordon, 2011). At its core the planning, analysis, execution and evaluation of social marketing programs is designed to influence the behaviour of target audiences (Andreasen, 2003; Lefebvre, 2013; Rundle-Thiele, 2015). In recent years social marketing has been used to increase healthful eating behaviour, mainly among children and adolescents (Keihner et al., 2011; Rosi et al., 2016; Young, Anderson, Beckstrom, Bellows, & Johnson, 2004), with relatively few interventions focusing on targeting young adults (Carins & Rundle-Thiele, 2014b). Andreasen’s (2002) social marketing benchmark criteria offers a useful guide to specify the extent that social marketing is employed within a change intervention and it includes six benchmarks namely behavioural change, formative research, segmentation, the use of marketing mix, exchange and competition. The National Social Marketing Centre (NSMC) extended the benchmark criteria by including eight elements, namely behaviour change, audience orientation, insight, segmentation, exchange, marketing mix, theory and competition (French & Blair- Stevens, 2006). This research includes consumer orientation, insight and healthful eating behaviour and is focussed on segmentation and theory. Segmentation is a process that groups target audiences into somewhat homogenous subgroups, or segments, and handles each segment separately based on the wants, needs and behaviours of the segments (Moss, Kirby, & Donodeo, 2009). Research shows that social marketing is more effective when more of the social marketing benchmarks are used (Carins & Rundle-Thiele, 2014b) and that segmentation is one of the least applied benchmarks (Kubacki, Rundle-Thiele, Lahtinen, & Parkinson, 2015). Segmentation discovers patterns of needs, wants and behaviours among groups within one target population and according to segmentation theory, different strategies should be tailored to each group to extend uptake (Donovan & Henley, 2010). Researchers have found that a better outcome may be produced when a program includes different strategies designed for different segments providing solutions that are more closely aligned to segment needs and wants (Dietrich, Rundle‐Thiele, Leo, & Connor, 2015). Theory offers a structured framework that can guide researchers and practitioners in the development and implementation of social marketing programs. Researchers have found that theory use is essential to accomplish successful programs (Eagle et al., 2013; Lefebvre, 2000). However, research shows that theory use in social marketing is rare (Luca & Suggs, 2013; Truong, 2014; Truong & Dang, 2017) and when theory use is reported its application is mainly weak (Pang, Kubacki, & Rundle-Thiele, 2017; Willmott, Pang, Rundle-Thiele, & Badejo, 2019). This thesis aims to test the replicability of theory-driven segments for healthful eating in young adult samples. This thesis begins by understanding the previously used segmentation bases and how (if at all) theory has been applied in social marketing healthful eating studies. Additionally, this thesis empirically tests the Motivation, Opportunity and Ability (MOA) framework in two young adult healthful eating contexts. More specifically, the aim of this research is to test if theory-driven healthful eating segments emerge from two young adult samples and to understand how each segment identified can be engaged to participate in a healthful eating program. To accomplish the aims of this research, two studies were conducted. Study 1 examined whether unique theory-driven segments emerged from a young adult sample. This study aimed to investigate which segmentation bases have previously been used to inform segment solutions; and apply the MOA framework (Rothschild, 1999) to determine if segments are evident in a young adult population. Post-hoc segmentation was used to establish the number and characteristics of segments with two-step cluster analysis. The findings from Study 1 indicate that the MOA framework can be used to explain healthful eating behaviour and two distinct segments were revealed with education, motivation to eat healthily and the Turconi eating behaviour score being the most important variables in segment formation. This study provides evidence of the value of including behavioural theory in segment formation. Study 2 was an empirical replication study that aimed to investigate to what extent similar healthful eating segments can be derived in two young adult populations. In line with replication principles Study 2 followed procedures applied in Study 1. The same segmentation variables were used to establish the number and characteristics of the segments. The findings from Study 2 indicate that the MOA framework once again explains eating behaviour in the second young adult sample and similar to Study 1 education was the most important variable. While the segment structure is similar to Study 1, differences were clear. Notably, the number of segments increased from two to three. The key outcome of this study is that care should be taken for segment identification given the number of segments differs in two different young adult populations. This research contributes to literature in four ways. First, this thesis identified segmentation bases that have previously been used by social marketers to derive segments and it identifies theories that have previously been used to inform segment solutions. Second, this thesis delivers a clear case study demonstrating that theory-informed segments exist within target populations. Application of a theoretically derived segmentation process can identify groups that have demonstrably different needs, wants and behaviours delivering actionable insights to inform program planning. Third, this thesis demonstrates the importance of generating a segmentation based profile in new young adult target populations given that differences emerge. Finally, this thesis delivers a practical contribution establishing how the segmentation process can be used to understand how each segment can be engaged to participate in a healthful eating program. Limitations and futures research directions are outlined.