The Houston Independent School District (HISD) has provided free prekindergarten classes for eligible Houston area four-year old students since the 1985-1986 school year. HISD offers full-day prekindergarten programs to all eligible students within its attendance boundaries. HISD collaborates with four federally-funded Head Start agencies that serve Harris County children located within the HISD boundaries: AVANCE Houston, Inc. (AVANCE); Gulf Coast Community Services Association (GCCSA); Harris County Department of Education (HCDE); and Baker Ripley (formerly Neighborhood Centers, Inc. (NCI)). The purpose of this study was to measure the effectiveness of PreK and Head Start Programs in HISD using comparative analyses of the performance of HISD kindergarten students who were enrolled in an HISD prekindergarten, Head Start or HISD-Head Start affiliated program during the 2015-2016 school year. Key findings include: (1) Students who were dually enrolled during the 2015-2016 school year in a prekindergarten program and were later enrolled in HISD kindergarten in 2016-2017 had higher mean standard scores than their Head Start Standalone peers on the 2016 Iowa Assessments English language arts (ELA) (123.6 vs 122.3) and math (121.8 vs. 120.8) as well as on the Logramos language arts (LA) (167.8 vs. 165.7) and math (163.5 vs 161.8) subtests; (2) The gap in performance between economically-disadvantaged and non-economically-disadvantaged students was either small or in favor of economically-disadvantaged ELA Head Start Standalone and dually enrolled and Head Start Standalone math students who took the Iowa Assessments subtests; (3) The gap in performance between economically-disadvantaged and non-economically-disadvantaged students was either small or in favor of economically-disadvantaged dually enrolled, HISD-PreK, and non-HS/HISD-PreK who took the Logramos LA and math subtests and Head Start Standalone students who took the Logramos LA subtest; (4) Dually-enrolled AVANCE (125.4), HCDE (122.1), and Baker Ripley (formerly NCI) (123.2) students had mean standard scores on the Iowa Assessments ELA subtest that were higher than those of their Head Start Standalone peers (121.4, 121.0, and 122.5, respectively); and (5) Economically-disadvantaged students who were enrolled in HCDE Head Start had a higher mean standard score compared to the non-economically disadvantaged peers on the 2016 Logramos LA and Math subtests.