Engineers choosing to change careers and become mathematics teachers, are a special group regarding their previous work experience and their math learning in the context of engineering. The research aim is to explore the mathematical understanding perceptions of engineers which chose to study mathematics teaching as a career change. Three engineers in the beginning of their teaching studies were interviewed in this qualitative preliminary research. Our preliminary results suggest that engineers at in the turn to mathematics education hold complex and varied perceptions of mathematical understanding. Their perceptions are presented and discussed in relation to relevant theory.