Children who attend a prekindergarten (Pre-K) program generally score higher on academic, social-emotional, and cognitive assessments at the start of kindergarten than children who do not. However, Pre-K nonattenders typically catch up to Pre-K attenders—sometimes partially and sometimes fully—by the end of kindergarten or first grade. Pre-K attenders still tend to outperform nonattenders in longer-term outcomes such as high school graduation, college attendance, and health status. But with the majority of American children scoring below proficiency in critical reading and math skills in elementary school, there has been considerable research and policy attention paid in recent years to what factors best sustain the Pre-K boost in the elementary school years. Drawing on four papers that use data from students enrolled in the Boston Public Schools (BPS), this brief explores evidence from testing three interconnected theories on how to sustain Pre-K benefits: skill type, sustaining environments, and instructional (mis)alignment.