Whether it is undertaken as a matter of choice, or has been imposed by external circumstances, career change is now commonplace. Career changers are a significant resource for educational authorities faced with a growing demand for teachers, especially specialist secondary school teachers. This qualitative investigation aimed to explain and understand the reasons and experiences of nineteen women and men who had embarked on new secondary school teaching careers. These career changers had previously pursued careers in occupations as diverse as accounting, scientific laboratory research, piloting international aircraft, and dance and drama studio teaching before choosing their new secondary school teaching careers. ‘Becoming’ as an explanatory grounded theory was constructed using the empirical data provided in their career change stories and experiences. This approach provided qualitative perspectives that have been largely absent in the larger body of quantitative studies of career change into teaching. Also under-represented in past research are investigations into career changers who undertake teacher training in a regional university with the specific aim of entering new careers as secondary school teachers, both aspects that further contextualised this investigation and its distinctive contributions to knowledge. Located within an interpretive constructivist paradigm, this investigation drew on the central principles and approaches of a grounded theory methodology to obtain rich data for analysis. Having recently completed an accredited pre-service teacher education program and taken up their first teaching appointments, these teachers shared their career change reasons and experiences through semi-structured, informal interviews from which empirical data were coded and categorised. As the central core concept of the explanatory grounded theory, ‘Becoming’ emerged through iterative inductive reasoning. It is underpinned by four contributory categories: The Attractions and Past Experiences, that illuminated the participants’ reasons for their career change decisions, and their attractions to secondary school teaching as their new careers; and, The Journey to a New Career and Developing a New Professional Identity. These categories were constructed from their experiences as students in the pre-service teacher education program they had completed in the year prior to participating in this research. This program offered them flexibility in study load and study mode; opportunities for professional collaboration; interactions with influential mentors; and authentic learning experiences as trainee teachers. ‘Becoming’ is a situation specific explanatory grounded theory. Its contribution to knowledge exists in its qualitative approach, and its specific focus on secondary school teaching. The investigation has identified the need to understand the circumstances surrounding career change decisions; the decision-making influences arising from positive and negative past experiences and antecedents; and the need for a flexible teacher education program that facilitates ‘Becoming’ a new career secondary school teacher.