It is our contention that research plays a role in creating and reifying the very concepts and notions that it seeks to study. Published research in multilingual mathematics education was reviewed to critically examine its underpinning rationality on: (a) how the learner is portrayed; (b) how mathematics education (teaching and learning) is portrayed; and (c) the notion of language as formulated in the texts. Two research trends were examined: large-scale studies on mathematics achievement and how they address multilingualism, and small-scale, classroom-based studies and their recommendations about practice. On the basis of this critical review we argue that research very often has a double effect of power. On the one hand it reifies categories of exclusion such as “multilingual learner” by documenting its existence. On the other hand, it provides methods and instruments to diminish the achievement gap and help the multilingual learner assimilate with the culture and language of the dominant group. We conclude that research rationality cannot be seen without a deeper questioning of the philosophical, ontological, and epistemological assumptions that underpin the traditional views of what constitutes mathematics and, by implication, mathematics education.