As open pit mines approach their economic limits, there is increasing impetus to consider transitioning to underground mining, either in parallel or as a subsequent phase to open pit operations. Various authors have developed approaches to optimise the transition from open pit to underground mining (Whittle et al, 2016; MacNeil and Dimitrakopoulos, 2017) with acknowledged limitations to define the preferred transition point. This paper explores an integrated optimisation approach that enables mining decision-makers to assess the full spectrum of technical (all mining methods including underground caving), economic (revenue, capital and operating costs), and operational factors (mine sequencing, production rates, ESG targets and constraints) influencing open pit to underground transition decisions. Further expanding on conventions through extensive practical experience, case studies and advanced modelling techniques, the authors demonstrate the substantial impact that an integrated approach can deliver.