Project Clarion is a five-year Javits project focusing on scale-up of science curriculum in the kindergarten through third grades. Eight curriculum units were developed. Each unit was structured to interweave: conceptual understanding, scientific reasoning and the investigative process, and content attainment. The units were designed for high ability learners to ascertain whether this curriculum may be used to benefit all learners, including students in Title I schools. Three curriculum-embedded performance-based assessments and rubrics were developed to measure conceptual understanding, science process skills, and content attainment. Results indicated that experimental students showed significant and educationally important performance gains.