Most children enter kindergarten when they are 5 years of age and move into first grade when they are 6. This time period is marked by great developmental change (Sameroff and Haith 1996), and children differ in what they can and cannot do socially, physically, and cognitively. Therefore, parents and educators are concerned whether certain children will have the knowledge and skills at age 5 to succeed in kindergarten. Over the years, policies and practices have emerged that are intended to improve children's early school experiences by giving them more time to develop and mature (e.g., changing age of entry requirements, transitional grades, readiness testing). Two such kindergarten enrollment strategies are retaining children for a second year of kindergarten and delaying the start of their first year of kindergarten. This report uses data from the Early Childhood Longitudinal Study, Kindergarten Class of 1998-99 (ECLS-K) to examine the relationship between kindergarten enrollment status (e.g., repeating kindergarten or delaying entry into kindergarten) and children's spring first grade reading and mathematics achievement. (Contains 16 endnotes, 1 figure, and 6 tables.)