The failure to maintain a healthy lifestyle (particularly consuming a healthy diet and engaging in regular physical activity) is a significant contributor to overweight and obesity and the resulting detrimental effects on individual and public health. Using Social Cognitive Theory and the Health Action Process Approach to inform the theoretical framework, this research examined whether maintaining a healthy lifestyle is influenced by different factors in overweight or obese and normal weight groups. Participants were 374 University students, categorized as overweight or obese and normal weight based on BMI determined by self-reported height and weight. They were assessed on measures of healthy eating and physical activity (at three-month intervals, to determine the maintenance of those behaviours over time) and variables predicted to influence those behaviours including: action self-efficacy, outcome expectations, intentions, planning, recovery self-efficacy, facilitators (i.e., consideration of future consequences, perceived social support and perceptions of general health) and impediments (i.e., depression, perceived stress and shame). Structural Equation Modeling was used to determine the fit of the predicted model for the total sample and for the overweight and obese and normal weight subgroups. The results demonstrated that a revised model predicting exercise behaviour (but not healthy diet) was able to adequately explain the data for the overall sample and for the normal weight subsample, but not for the overweight and obese subsample. Outcome expectations about exercise, self-efficacy and planning made important contributions to the prediction of exercise behaviour, but not diet. Implications of these findings will be discussed.