The main part of this dissertation study consists of a survey of the types of articles (e.g., articles based on qualitative research, articles based on quantitative research, theoretical articles, etc.) that appear between 2016 and 2021 in five of the leading mathematics education journals (N = 1,317). In addition, for the articles that carry out quantitative research (N = 115), and the studies examined in one of the articles that reports on a meta-analysis (N = 22), the author examines the variety of variables studied, along with the measures used to study these variables and the methods of analysis used. The author hopes that the results of this work can shed some light on the current nature and progress of the domain of Mathematics Education Research (MER). In pursuit of this goal, a theoretical framework labeled Action Domain Theory (ADT) is developed. The basic premise of ADT is that, to any kind of organized effort (referred to as a domain), such as MER, there is a goal (or set of goals) that drives all actions taken in the domain. In the context of the domain of MER, the author hopes that these explorations can provide some clues pointing towards the goals of the domain, and perhaps also the extent to which the current research is moving towards some of these goals. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]