Act 166 (Universal PreK) was enacted on May 10, 2014. Universal PreK is jointly administered by Agency of Education (AOE) and Agency of Human Services (AHS), Department for Children and Families (DCF). In 2015-16, approximately one-third of Supervisory Unions and Supervisory Districts (SUs/SDs) implemented Universal PreK with two-thirds waiting until 2016-17 to begin full implementation. Many systems that decided to delay full implementation continued to voluntarily offer publicly funded prekindergarten authorized under pre-existing law. School systems that fully implemented Act 166 in 2015-16 were called ‘early adopters’. Only one third of school systems implemented Universal PreK that year. The data in this report reflect PreK participation in the 2017-18 school year, which represents the second year of full implementation of Act 166. Data quality are still in need of improvement, and therefore caution is advised in drawing conclusions based on these first years of PreK implementation. (author abstract)