Parental involvement in interventions is key to their success. Drawing on data from an ongoing book-based parenting intervention aimed at increasing knowledge of child development among fathers and mothers of infants, we examined parents’ participation and quality of their engagement in the first 2 waves of the intervention, when children were 9 to 12 months old. We also examined the factors that predicted parents’ level of participation in the intervention. We report 2 sets of findings. First, parents participated an average of 2.6 times per week, and mothers participated more frequently than fathers. Almost all parents reported that they enjoyed reading the books regardless of their level of participation, though mothers reported slightly more enjoyment than fathers. Second, results of regression tree analyses showed that the most important predictor of mothers’ and fathers’ participation in the intervention was whether or not their partner was also participating. The other important set of predictors was the level of resources, hours worked, education, and household income for fathers and employment status and income for mothers. Our findings have important implications for improving fathers’ and mothers’ participation in interventions.