There is growing international research evidence about young children's engagement with mathematics education. However, much of this evidence is drawn from children aged four years and upwards. This paper reports findings from a systematic review of peer-reviewed research concerning mathematics education for children aged "under" four. The majority of the studies took place within an early childhood education service such as a preschool or day care centre, and task-based interviews and standardised assessments were most commonly used. Most of the papers focused on either educators' knowledge, attitudes and strategies, or children's mathematical competencies. The literature presents compelling evidence that children "do" engage in mathematics education prior to four years of age, and that they possess many mathematical competencies. Findings suggest that educators play a critical role in shaping the mathematical learning opportunities available to children; however, there is some uncertainty among educators about how to support young children's mathematics learning.