Early childhood education (ECE) programs have been shown to produce immediate positive impacts on children's cognitive abilities, academic knowledge, and social-emotional skills. However, some research suggests that impacts may be modest and short-lived. That is, even though ECE enables participating children to begin kindergarten with greater skills on average compared with their peers, the skills of ECE attendees and nonattendees appear to converge as children progress through school. Thus, any initial differences between these groups observed at school entry are reduced or eliminated over time, a phenomenon that has been described as "fade-out," "catch-up," or both. This systematic review assesses our current understanding of the conditions under which ECE impacts persist or fade over time, which is critical because of the potential intervention and policy implications. Recent work has begun to make progress in this direction, but future efforts that address the present gaps and limitations of the field are needed in order to maximize the long-term impacts of the next generation of ECE programs. Public Significance Statement This systematic review examines when and why the impacts of early childhood education programs fade or attenuate over time. Specific mechanisms and moderators of persistent impacts are identified, and directions for future research that can shape practice and policy are discussed.