This cluster-randomized design with two 2-year cohorts is designed to test the efficacy of an early childhood science education curriculum (Head Start on Science; HSOS) and professional development program in improving teacher science practice and child school readiness. Participants include a large, diverse group of 603 low-income children from 71 classrooms and 149 teachers. The intervention included professional development in the Head Start on Science Curriculum and distance coaching throughout the school year. All data collection is complete and data are being coded. Preliminary results show significant developmental changes in children’s use of science processes and practices. Tests of the HSOS intervention suggest some changes, but require more data coding and analysis prior to final conclusions.