The purpose of this study was to evaluate the association between children's academic and social-emotional skill levels at entry into Head Start (HS) and their subsequent academic growth through HS and into kindergarten. We first examined HS children's growth trajectories in math, reading, and receptive vocabulary skills over a period of 2.5 years (i.e., between HS entry and kindergarten). Then, we examined whether children's capabilities in academic and social-emotional skills at HS entry were associated with their academic growth trajectories. The study was guided by two competing theories on the effectiveness of early care and education (ECE) programs, the "skills-beget-skills hypothesis" and the "compensatory hypothesis." A sample from the Head Start Family and Child Experiences Survey 2006 Cohort (FACES 2006) was analyzed using three-level growth curve modeling. Children who had lower receptive vocabulary skills at HS entry showed faster growth in receptive vocabulary skills. This result supports the compensatory hypothesis, which suggests that quality ECE programs have larger program effects for more disadvantaged children. For math and reading skills, no association between children's entry-level skills and their growth rate was found. Social-emotional skills at HS entry were positively associated with either concurrent baseline academic skills or their growth rate over time, partially supporting the skills-beget-skills hypothesis, which posits that the skills children possess before an intervention allow them to better acquire program benefits. (author abstract)