A large-scale population trial using the Triple P-Positive Parenting Program (TPS) was evaluated. The target population was all parents of 4- to 7-year-old children residing in ten geographical catchment areas in Brisbane (intervention communities) and ten sociodemographically matched catchment areas from Sydney (5) and Melbourne (5), care as usual (CAU) comparison communities. All five levels of the Triple P multilevel system of intervention were employed; including a local mass media strategy, a primary care strategy, and three more intensive levels of parenting intervention delivered by a range of service providers (e.g., health, education, and welfare sectors). Program outcomes were assessed through a computer-assisted telephone interview of a random sample of households (N = 3000) in each community at pre-intervention and again at two years post-intervention. At post-intervention there were significantly greater reductions in the TPS communities in the number of children with clinically elevated and borderline behavioral and emotional problems compared to the CAU communities. Similarly parents reported a greater reduction in the prevalence of depression, stress and coercive parenting. Findings show the feasibility of targeting dysfunctional parenting practices in a cost-effective manner and the public acceptance of an approach that blends universal and targeted program elements. Editors' Strategic Implications: This is the first positive parenting program to demonstrate longitudinal, population-level effects for parents and children. The authors provide an excellent example of multilevel prevention planning, coordination, execution, and evaluation.