The study reported in this book focused on teachers'' task practices in relation to teachers'' knowledge. The current mathematics curricula around the globe call for learning experiences, particularly tasks that support learners'' active participation in developing mathematical ideas in the classroom. Tasks differ in qualities and possibilities for learners'' participation and thus, teachers regularly make choices in terms of tasks for learners to work on. The criteria that inform teachers'' choices of tasks, how teachers and students work with the selected tasks in class, how teacher knowledge is implicated in what teachers do with tasks and how task practices relate to learning achievement were the focus of the study. The data reported were from 11 Grade 10 and/or Grade 11 teachers, from 7 schools in Lagos State, Nigeria, as they taught geometry. Data collection techniques/instruments included teacher knowledge interviews, pre-lesson interviews, video recorded lesson observations, post-lesson reflection interviews, students'' tests, copies of teachers'' and learners'' notes and workbooks. Important findings were reported in this book and recommendations were made as appropriate.