Although approximately 5-8% of students have a mathematical learning disability (MLD), researchers have yet to develop a consensus operational definition. To examine how MLD has been identified and what mathematics topics have been explored, the authors conducted a systematic review of 165 studies on MLD published between 1974 and 2013. To move the field toward a more precise and shared definition of MLD, the authors argue for standards for methodology and reporting, and they identify a need for research addressing more complex mathematics.