As one of the three Rs, "'rithmetic" has always been central to education and education research. By virtue of that centrality, research in mathematics education has often reflected and at times led trends in education research. This chapter provides some deep background on epistemological and other issues that shape current research, with a primary focus on empirical research, which sprouted and flowered over the past 100 years or so--roughly coinciding with the existence of the American Educational Research Association as a professional organization. The author begins by tracing the growth and change in research in mathematics education and its interdependence with research in education in general over much of the 20th century, with an emphasis on changes in research perspectives and methods and the philosophical/empirical/disciplinary approaches that underpin them. He then turns to an overview of currently flourishing research and some indications of potentially productive arenas for future work.