Marine renewable energy, including tidal renewable energy, is one of the less exploited renewable energy sources that could contribute to energy demand while reducing greenhouse gas emissions. Several proposals to build tidal range structures, e.g. Swansea Bay Lagoon (SBL), have not received support from the UK government due to the high electricity costs or uncertainty about the environmental impacts. This makes the optimisation of such schemes particularly important for the future. The aim of this research was to optimise the design and operational characteristics of Tidal Range Schemes (TRSs) to make them more economically attractive by maximising the energy generation, or a flexible energy output to achieve multi-objectives. The study has focused on two key issues of TRSs optimisation. Firstly, the majority of studies before adopted the traditional non-flexible operation scenarios for electricity generation. In this approach, the operation heads were fixed throughout the operation simulations. It ignores the variability of tidal range over time and the fact that the operation of each generation phase affects the water levels inside the basin which in turn impact the electricity generation of the next phase. Secondly, the flexibility of energy output provided by renewable energies including tidal energy was underexploited, but it is regarded as one of the most important parts of the UK’s energy mix. Hence, the first objective was to propose and optimise flexible operation schemes to maximise energy generation. To achieve this, optimisation approaches were considered by breaking the operation into small components to optimise the operation of TRSs using a widely used 0-D modelling methodology. The optimisation outcomes were verified by a 2-D unstructured model under the same conditions. The flexibility of operation could at least increase generated electricity by 10% compared to the traditional non-flexible head operation. This increase was further improved by at least 10% when pumping was included. Meanwhile, a Genetic Algorithm (GA) method used for flexible operation optimisation was able to achieve the same amount of electricity generation compared to using a Grid Search (GS) method. However, the GA model could save approximately 50% of the computational cost, and it could be 95% in the optimisation of multiple variables, e.g. design parameter combining with flexible operation. Additionally, the optimisation using GA was used in designing of the two of the biggest lagoons proposed in the UK, namely West Somerset Lagoon and North Wales Tidal Lagoon, with the energy generation of 5.57 TWh/Year and 4.81 TWh/Year, respectively. The second objective in this study was to achieve the flexible energy output optimisation, including utilising generation flexibility from multilagoons to help match the continuous trends of energy output. The flexible operation optimisation was proved to facilitate better utilisation of renewable energy through the development of TRSs for multiobjective decision making.