The National Assessment of Educational Progress (NAEP) has regularly assessed the reading skills of U.S. fourth-graders since the early 1970s, but few national studies have assessed the reading skills of children when they enter kindergarten and then documented the development of these skills through fifth grade. This analysis uses data from the Early Childhood Longitudinal Study, Kindergarten Class of 1998-99 (ECLS-K) to describe children's home and classroom literacy environments and discuss how factors such as home literacy environment and length of school day relate to children's reading achievement. Findings from the analysis reveal that the differences in children's reading skills and knowledge usually seen in later grades appear to be present as children begin school, and that the resources children possess when they start kindergarten--such as their early literacy skills and the richness of their home literacy environment--are related to their reading proficiency across kindergarten and first grade. (Contains 8 figures and 8 endnotes.)