The chapter documents the emergence of mathematics education as a field in its own right, with its own distinctive theories, methodologies, and preoccupations. We present four widely differing examples of theoretical/practical programs of work that illustrate the rich diversity that now characterizes the field. These examples reflect a considerable maturation of the field, in terms of disciplinary influences, methodologies, philosophical and epistemological analyses, and broader considerations of the roles and purposes of mathematics education as embedded in historical, cultural, and societal contexts.