The study that we report in this chapter contributes to our broader research agenda for evaluating the impact of a national professional development programme that upskills out-of-field post-primary mathematics teachers in Ireland. The aim of the study was to compare the self-efficacy beliefs, perceived and observed classroom practices of six post-primary mathematics teachers (three groups of 2) who were either out-of-field, upskilled via the professional development programme, or in-field. The teachers completed surveys of their self-efficacy beliefs and approaches to teaching mathematics. Video recordings of three mathematics lessons taught by each teacher were analysed using the Productive Pedagogies classroom observation framework. The findings showed that there were similarities and differences between the three groups of teachers; however, the upskilled teachers were developing self-efficacy beliefs and pedagogical practices that are similar to those of in-field teachers of mathematics.