This study examined dosage effects of multiple intervention programs on young children's language, literacy, and general development. By employing a hierarchical linear model on a sample of 1,436 children from 72 classrooms, developmental outcomes of four-year-old children simultaneously receiving differing numbers of intervention services were compared: Group 1: Pre-Kindergarten only, Group 2: Pre-Kindergarten + Early Reading First, Group 3: Pre-Kindergarten + Early Reading First + Head Start. Additionally, the effects of previous participation in Early Head Start/Head Start before age four as well as the effects of classroom quality on child outcomes were examined. Findings suggested that understanding the primary focus areas of each program was a key to understanding the dosage effects of multiple intervention programs on children's developmental skills. Also, integrating additional intervention programs within a limited time frame may not necessarily lead to greater gains in child outcomes because positive and negative effects of a specific program might negate the effects of other programs. Having additional days of participation in Early Head Start and/or Head Start before age four positively predicted the developmental gains children made at age four in the areas of physical and cognitive development. A pattern of relationships between classroom quality and child outcomes revealed a close alignment between quality features and the particular child outcomes considered. Policy implications regarding the appropriate level of intervention dosage were discussed. (author abstract)