This technical summary describes the changes in science performance on exercises included in both the first and second science assessments and on exercises included in both the second and third science assessments conducted by the National Assessment of Educational Progress (NAEP). Using the same exercises for adjacent assessments, with some exercises common to all three, National Assessment was able to measure improvements and declines in achievement between 1969-70 and 1976-77. Each assessment utilized a deeply stratified, multistage probability sample design and a professional data collection staff. To the extent possible, administration conditions were kept constant across assessments. The document contains a brief introduction and four chapters: chapter 1 contains background information about the project, chapter 2 presents national results by age levels (9 years, 13 years, 17 years), chapter 3 reports group results for 9-, 13- and 17-year olds, and chapter 4 contains a discussion of the adult science assessments. Four appendices are included: (1) A-Technical Procedures: Sampling and Estimation of Standard Errors; (2) B-Estimated Population Proportions of Reporting Groups Based on National Assessment Samples, 1969-70, 1972-73, and 1975-76; (3) C-Changes in Procedures Between Assessments; and (4) D-Nonresponse in Assessment Samples. (PEB) Primary type of information provided by report: Results (Technical Summary).