Over the last decade Finnish students’ performance on the mathematical literacy components of PISA has created much international interest. However, with respect to the two times Finland has participated in the Trends in International Mathematics and Science Study (TIMSS), Finnish students’ mathematical performance has painted a very different picture, particularly at grade 8. What is less well known is that Flanders, whose Programme for International Student Assessment (PISA) achievements have been masked by those of Belgium as a whole, has performed as well as Finland with respect to mathematical literacy and, on the three TIMSS in which it has participated, it has been the most successful European system at grade 8. Thus, while Finnish performance on tests of technical competence, despite success on tests of mathematical applicability, has been moderate, Flemish students have led the Europeans on both. In this chapter, the author examines two sequences of videotaped lessons taught on percentages, a topic resonant with ambitions of both technical competence and mathematical applicability, by case-study teachers considered against local criteria to be effective. The evidence suggests that Finnish mathematics didactics are more likely to explain Finnish TIMSS failure than PISA success. Flemish didactics may have greater explanatory potential for both PISA and TIMSS success. Such findings suggest that performance on international tests of achievement may be unrelated to didactical quality as other, typically hidden, cultural factors intercede.