Globally, teacher professional development is heralded as a key mechanism for educational reform. With governments investing heavily in PD programs, the aim of these interventions is not only enhanced teacher knowledge and practice but, ultimately, improved student outcomes. A substantial body of research has attempted to identify characteristics of effective PD, generating a growing list of features that ostensibly 'work'. As such, program design has become the dominant analytic focus. In this paper, we shift attention squarely to program implementation as necessary in conceptualising and evaluating effective PD. We apply the lens of implementation science to a case study of how one regional secondary school in NSW, Australia, implemented a robust PD program called Quality Teaching Rounds that has strong evidence of effectiveness. Despite the school's attempts to remain true to the spirit of the PD, a combination of remoteness, lack of casual relief teachers, high teacher turnover, and negative perceptions of peer observation result in a form of QTR that is almost unrecognisable from its intended design. We argue greater attention must be given to understanding and supporting successful implementation within and across diverse school contexts in order to take effective forms of PD to scale.