There is broad conceptual consensus that the effectiveness and efficiency of early childhood intervention (ECI) for vulnerable children is correlated with the active involvement and full participation of parents. This requires a general understanding of each family's needs and desires and specific strategies for determining what each family needs to achieve the outcomes they desire. However, the effect sizes of parent programs or involvement--due to methodological issues and mediator variables--still vary significantly between the programs. The author highlights the importance of shared understanding and shared decision between professionals and parents as a main predictor of high outcome in ECI.