Based on student, teacher, and school data from two large-scale assessments conducted in 1970/71 and 1983/84 gender differences in science achievement are analyzed across 10 countries. Though concentrating on the age group of 14-year-old students, selected results are also presented for 10-year-olds and students from the terminal secondary school level. Using state-of-the-art multilevel multivariate statistical techniques gender differences are analyzed across the two data points, countries, age groups and science subjects. Data were available for about 400,000 students, 50,000 science teachers and 15,000 schools world-wide from the two IEA international science studies. Complex path models are discussed for each of the countries showing the network of factors found to influence the manifestation of gender differences in science achievement and special analyses implementing hierarchical linear modelling techniques are employed for Australia and England, for which single-sex schooling is compared with the more common form of coeducation.