Research has demonstrated that teacher identity matters in mathematics education. This is of heightened concern when we consider those teaching mathematics out-of-field, a phenomenon prevalent at the post-primary level in the Irish context. A national program (PDMT) to upskill out-of-field teachers was established and current research is appraising graduates’ experiences. In this chapter, we bring together out-of-field teachers’ knowledge and identities, using Kelchtermans’ (2009) concept of professional self-understanding, which is an essential part of a teacher’s personal interpretive framework and acts as a lens through which teachers view their job, give meaning to it and act in it. We report on aspects of an online, primarily quantitative, survey administered to graduates of the PDMT examining their professional self-understandings on completion of the programme. The findings contribute to our understanding of important considerations relating to the development of professional learning programmes for upskilling out-of-field mathematics teachers.