Objective: To examine the relationship between health belief model (HBM; i.e., perceived threat, benefits, costs, and self-efficacy) and theory of planned behavior (TPB; i.e., attitudes, social norms, and perceived behavioral control) constructs and parents' intention to participate and initial engagement (i.e., recruitment, enrollment, and first attendance) with a parenting intervention. Method: Participants were parents (n = 699, mean age = 38.29 years, 90.4% mothers) of 2–12-year-old children. The study conducted secondary analysis of cross-sectional data collected for an experimental study of engagement strategies. Participants provided self-report data on HBM constructs, TPB constructs, and intention to participate. Measures of initial parent engagement were also collected (i.e., recruitment, enrollment, and first attendance). Logistic regressions evaluated the impact of HBM and TPB constructs, and their combination, on intention to participate and initial parent engagement. Results: Analyses indicated that all HBM constructs increased the odds of parents' intention to participate and enrollment. In terms of TPB, parents' attitudes and subjective norms, but not perceived behavioral control, were significant predictors of intention to participate and enrollment. When combined in one model, parents' perceived costs, self-efficacy, attitudes, and subjective norms predicted intention to participate, whereas perceived threat, costs, attitudes, and subjective norms increased odds of enrolling in the intervention. Regression models for first attendance were not significant and those for recruitment could not be conducted due to lack of variance. Conclusions: The findings demonstrate the relevance of using both HBM and TPB constructs when enhancing parent intention to participate and enrollment. What is the public health significance of this article?: This study suggests that health behavior theories play a different role across stages of parents' initial engagement with parenting interventions. Both health behavior theories inform elements to be included when implementing targeted engagement strategies for each stage of initial engagement, particularly intention to participate and enrollment in parenting interventions. [ABSTRACT FROM AUTHOR]