Early Childhood Caries (ECC) is one of the most prevalent chronic diseases in childhood, and a key public health burden affecting preschoolers worldwide. Despite being a highly preventable disease, almost half of Australian 5-year-olds have dental caries experience. Parents are recommended to perform twice-daily parental supervised toothbrushing using fluoride toothpaste until children are at least 5 years of age. Parental involvement plays a central role in the development of oral self-care habits with the preschool years offering a crucial period for building and transferring lifelong good oral hygiene behaviours. However, the high prevalence of ECC, along with recent self-report evidence, indicates parents are not meeting the recommended preventive behavioural guidelines of twice-daily parental supervised toothbrushing for their preschool-aged children. This highlights a pressing need for theory- and evidence-based strategies for promoting regular parental supervised toothbrushing among parents of preschool-aged children. The aims of this PhD program of research were to identify the social cognitive factors associated with the oral hygiene behaviours of preschoolers, and to apply the health action process approach (HAPA) to predict and promote regular parental supervised toothbrushing, investigated through three studies. Study 1 was a systematic review and meta-analysis aimed at identifying the parental social cognitive factors associated with the oral hygiene behaviours of preschoolers. Five bibliographic databases were searched. A study was eligible for inclusion when it reported an association between a parental social cognitive factor and an oral hygiene behaviour in the targeted age cohort. Meta-analyses were performed when there were at least four independent effect sizes (k>3). Of the 5,945 records identified, 25 studies contained eligible data to be included in four meta-analyses: attitude (k = 12), self-efficacy (k = 12), intention (k = 6), and sense of coherence (k = 5). The results showed that greater frequency of preschoolers’ oral hygiene behaviour was significantly associated with parental attitudes (r+ = .18), self-efficacy (r+ = .34), and intention (r+ = .29), and not significantly associated with parental sense of coherence (r+ = .08). Self-efficacy, attitudes, and intention were identified as significant correlates of parental-supervised oral hygiene behaviour. However, this is a limited evidence base and many social cognitive factors, such as self-regulatory processes including planning and action control, have yet to be explored in this context. Study 2 examined the social cognitive determinants of parental supervised toothbrushing guided by the HAPA. In a prospective correlational survey study, participants (N = 185, 84.3% women) completed measures of HAPA social cognitive constructs at an initial time-point (T1), and 12 weeks later (T2) self-reported on their parental supervised toothbrushing behaviour, toothbrushing habits, and action control. Structural equation models exhibited adequate fit with the data when past behaviour and habits were either excluded or included. Intention, self-efficacy, planning, and action control were predictors of parental supervised toothbrushing; intention predicted action planning and coping planning; and self-efficacy and attitude were predictors of intention. Indirect effects of social cognitive constructs through intentions, and intentions through planning constructs were also observed. Inclusion of past behaviour and habits attenuated model effects. Results indicate that parental supervised toothbrushing is a function of motivational and volitional processes. This knowledge can inform behaviour change interventions targeting parental supervised toothbrushing. Study 3 aimed to test the efficacy of a HAPA-based intervention to promote regular parental supervised toothbrushing among parents of preschool-aged children. We tested effects of an intervention employing information provision, behavioural instruction, implementation intention, and mental imagery techniques in a pre-registered study adopting a randomized controlled design in a sample of Australian parents of preschoolers (N = 254). Using an additive design, participants were randomly assigned to an active control arm, or one of the three experimental arms (self-efficacy+, planning+, or action control+). We aimed to test whether the additional treatment components present in the experimental treatment arms provided greater benefit, compared with the components that are held constant across the control and intervention arms (e.g., testing the full package of HAPA-based intervention components against components targeting only self-efficacy and planning; or self-efficacy alone). The intervention was delivered online, and participants completed measures of behaviour and HAPA-based social cognitive constructs pre-intervention and four-weeks post-intervention. Mixed-model ANOVA analysis revealed no intervention effects on behaviour; however, intervention effects for planning were observed in the two conditions that received the planning intervention: the action control+ and planning+ conditions. Furthermore, intervention effects favouring the action control+ condition were observed for increases to intention and self-efficacy. Overall, the findings of this PhD research program make an important contribution to the empirical knowledge on parental supervised toothbrushing and provide important theoretical and practical implications. The research contributes to the cumulative empirical evidence about motivational and self-regulatory components in health behaviour change; in particular, self-efficacy and planning. Present findings provide formative support for the core proposed effects among the motivational and volitional factors of the HAPA and shed light on their effects on parental supervised toothbrushing behaviour when targeted in an intervention.