In this article, the authors discuss two ways of improving the rigour and integrity of mathematics in STEM programs drawing on examples from their research. The authors examine how mathematics can be made a stronger part of a STEM unit using examples from two pairs of teachers engaged in a STEM teacher professional development program, conducted across two years in thirty schools with sixty teachers. The article discusses how teachers can start with the mathematics and build a STEM unit using an example from the reSolve: Mathematics by Inquiry project. In each example the authors provide links to the Mathematics Curriculum, but one of the joys of teaching mathematics within a STEM unit is that it addresses the curriculum in multiple and rich ways.