Mathematical tasks have long been recognized as crucial mediators between mathematical content and the mathematics learner. For many people, the mathematics classroom is defined by the type of tasks one finds there - and this is appropriate. Mathematical tasks are the embodiment of the curricular pretext that brings each particular set of individuals together in every mathematics classroom. In other contexts, individuals come together to engage in musical performances or dramatic performances. The performances of the mathematics classroom are largely the performance of mathematical tasks and if we are to understand and facilitate the learning that is the purpose of such settings then we must understand the nature of the performances that we find there. The classroom performance of a task is ultimately a unique synthesis of task, teacher, students and situation. Of particular interest are differences in the function of mathematically similar tasks when employed by different teachers, in different classrooms, for different instructional purposes, with different students. By making comparison possible between the classroom use of mathematical tasks in different classrooms around the world, the analyses reported in this book reveal the profound differences in how each teacher utilises mathematical tasks, in partnership with their students, to create a distinctive form of mathematical activity. The Learner's Perspective Study aims to juxtapose the observable practices of the classroom and the meanings attributed to those practices by classroom participants. The LPS research design documents sequences of at least ten lessons, using three video cameras, supplemented by the reconstructive accounts of classroom participants obtained in post-lesson video-stimulated interviews, and by test and questionnaire data, and copies of student written material. In each participating country, data generation focuses on the classrooms of three teachers, identified by the local mathematics education community as competent, and situated in demographically different school communities within the one major city. The large body of complex data supports both the characterisation of practice in the classrooms of competent teachers and the development of theory.