Time and acceptance are criterion often used to measure the legitimacy and worth of an idea. Two decades have passed since the first publications that introduced Teaching Games for Understanding (TGfU) as a means to conceptualize games teaching and learning. For over two decades various professionals have advocated for TGfU as a sound idea, which is built on assumptions about games education. We will outline why we believe that there is cause for celebration for TGfU as an innovation to games learning. Second, we will argue for the need to work toward legitimacy through data-based, not data-free development. Absent from current discourse are efforts to support assumptions about how students learn games while engaged in the TGfU approach. The case for legitimacy will only improve with more data-based development work. Field-based research needs to be an essential part of good development work thus leading us toward research-based practice. We should consider more programmatic research, which could be grounded in three possible robust theoretical frameworks: (a) achievement goal theory; (b) information processing; and (c) situated learning theory, which could have strong implications for games learning specifically as it relates to TGfU.