The goal of the Inquiry Synthesis Project was to synthesize findings from research conducted between 1984 and 2002 to address the research question, "What is the impact of inquiry science instruction on K-12 student outcomes?" The timeframe of 1984 to 2002 was selected to continue a line of synthesis work last completed in 1983 by Bredderman [Bredderman [1983] "Review of Educational Research" 53: 499-518] and Shymansky, Kyle, and Alport [Shymansky et al. [1983] "Journal of Research in Science Teaching" 20: 387-404], and to accommodate a practicable cut-off date given the research project timeline, which ran from 2001 to 2006. The research question for the project was addressed by developing a conceptual framework that clarifies and specifies what is meant by "inquiry-based science instruction", and by using a mixed-methodology approach to analyze both numerical and text data describing the impact of instruction on K-12 student science conceptual learning. Various findings across 138 analyzed studies indicate a clear, positive trend favoring inquiry-based instructional practices, particularly instruction that emphasizes student active thinking and drawing conclusions from data. Teaching strategies that actively engage students in the learning process through scientific investigations are more likely to increase conceptual understanding than are strategies that rely on more passive techniques, which are often necessary in the current standardized-assessment laden educational environment. (Contains 13 tables.)