The purposes of the present study were (a) to compare US and Korean 8th graders' mastery of knowledge and skills in the mathematics test of the Trends in International Mathematics and Science Study (TIMSS) 2003 using a cognitive diagnostic testing method and (b) to find links between teachers' instruction and students' mastery of mathematics knowledge and skills. The participants included 740 US and 439 Korean 8th graders who took the Booklet 3 mathematics test. The results showed substantial differences between the US and Korean students' performance in problem restructuring and reasoning, measurement, and geometry. The most helpful instructional strategy for both Korean and US students was encouraging students' independent problem solving. Reviewing, re-teaching, and clarifying content were especially effective for the US students. Implications for teaching and learning are discussed. (Contains 1 table and 3 figures.)