The CDC's Essentials for Parenting Toddlers and Preschoolers program (EFP) is a free Internet resource with the potential to break down barriers to population-wide access to scientifically-based parenting interven- tions. Our research team at NYU recently partnered with program developers and researchers at the CDC and Westat (a research firm) to evaluate EFP. We learned three important lessons from the pilot study: (1) EFP is well-liked by the parents who use it; (2) parents consume more intervention content if cued what content to use and when to use it; yet (3) the amount of content consumed, even with such guidance, is highly variable and tends to be quite limited. In short, EFP has considerable promise, but parental engagement, a major issue in the success of universal parenting interventions, remains a challenge. Thus, learning how to maximize meaningful parent engagement is the lynchpin to realizing the promise of EFP. The objective of the proposed research is to optimize EFP by identifying engagement-fo- cused intervention elements to add to EFP that enhance its effects on parenting skills. The study is based on the Multiphase Optimization Strategy (MOST) developed by Dr. Linda Collins (Co-I) and collabora- tors. The bedrock logic of MOST is that only after an intervention has been optimized to meet specific criteria, should it be evaluated in an RCT. In our view, improvements in parenting skills are the key optimization crite- ria that must be met. The study is a longitudinal factorial optimization trial in a community sample of 800 parents with 1.5- to 3-year-old children. There are 4 experimental factors; each corresponds to the presence vs. absence of an engagement-focused intervention element. This experiment enables the estimation of the indi- vidual and combined effects of each element. The specific aims follow. Aim 1: Optimize the effects of EFP on parenting skills by determining which combination of the four ex- perimental engagement-focused intervention elements results in the greatest success of EFP, as reflected in increasing parent warmth and reducing corporal punishment, overreactive and lax discipline. Aim 2: Determine the extent to which boosted meaningful parent engagement in EFP is the mechanism driving the effects of the four engagement-focused intervention elements on parenting skills, and which as- pect(s) of engagement (e.g., content consumption; behavioral skills practice) are the key mediators that trans- late the effects of the four engagement-focused intervention elements into improved parenting. Aim 3: Examine parent (e.g., change readiness; race) and child characteristics (e.g., externalizing behavior; sex) to determine if the optimal intervention package differs among subgroups. Cracking the code of providing parents with an intervention that they actually use, and that improves parenting, could have far-reaching ef- fects (e.g., improving population-level child outcomes).