This thesis presents a subset of the author’s published works, and describes the impact his work has had on ocean forecasting systems. This impact can be broadly divided into two themes, the underpinning development of ocean forecasting models and the implementation, tuning and evaluation of those models to ensure they provide skilful products, with value to users. The systems described in this dissertation are recognised as amongst the best available, and are being used by commercial operators, military decisions makers and governmental organisations, as well as research users. They also form the basis on which future systems will be developed, meeting the challenges and addressing the priorities discussed in the thesis. The author expects to have a substantive impact on driving the research agenda in these areas over the coming years.