There is substantial opportunity in the development of wind and tidal current turbines to help decarbonise national energy systems and to expand the economic potential of the European Union. However, well-documented cases of renewable energy device failures have highlighted the issue of reliability and the importance of durability in the structural design of blades. Furthermore, electricity generated by tidal devices is currently over twice the price of electricity generated by offshore wind turbines, due to the infancy of the technology, remote installation locations and short maintenance windows for these devices. A significant component of the total costs for both types of turbine is the high cost of rotor blade manufacturing (up to 15% of the turbine capital expenditure). Additionally, the long-term loading on tidal turbines is yet to be fully characterised, with the effects of fatigue damage an area of particular concern. There is a need to ensure the effective functioning of every component of the turbine and to minimise the associated costs. This thesis presents the application of numerical techniques and design optimisation strategies for wind and tidal turbine blades. The design studies contained in the thesis advance upon previous studies in this field by combining blade element momentum theory, damage and fatigue analysis methods and genetic algorithm design optimisation routines for composite turbine blades. The contributions made to the state of the art are relevant from both a scientific and an industrial perspective. At the core of this research is the development of an automated computational design and analysis methodology for wind and tidal turbine blades. The first iteration of the methodology included novel aspects such as: non-linear damage modelling of composite materials under static loading conditions, hydrodynamic modelling using blade element momentum theory (BEMT) and finite element (FE) sub-modelling techniques. The concept design of the blade for a 1 MW tidal turbine was performed, investigating the application of glass and carbon fibre-reinforced materials and the subsequent effects on blade stiffness and composite damage. The analytical capacity of the computational methodology for full-scale composite structures was confirmed against physical testing of a 13 m long glass fibre-epoxy wind turbine blade. Once the FE representation of the blade was validated, the methodology was combined with a genetic algorithm (GA) to optimise the mass of the blade. The automated nature of the methodology enabled the efficient variation of model parameters and a set of near-optimum blade designs were evaluated. The methodology was further developed to incorporate a multi-objective GA and an optimisation study was performed on 4.5 m long glass fibre-reinforced polypropylene blades. Experimental tests were carried out to characterise the blades and to calibrate the FE blade models. The optimisation approach was validated by testing a custom-made glass fibre blade and comparing the results to numerical models. Finally, the fatigue performance of a concept tidal turbine blade was examined. The BEMT model was enhanced and validated against published experimental results and applied to actual tidal current data to determine the fatigue loading on the blades. A post-processing code for assessing the damage to the composite material was also developed, based on constant life diagrams for representative glass fibre-reinforced materials, Miner’s rule and Rainflow counting. The fatigue design study incorporated the aspects of structural modelling and hydrodynamic modelling, together with the optimisation techniques developed throughout the thesis, to successfully design and analyse a full-scale composite blade. The automated computational design methodology allowed a variety of blade designs to be assessed before testing. Coupled with the optimisation approaches, the methodology showed significant mass reductions for wind and tidal turbine blade designs, while retaining the strength and stiffness of the composite structures. Mass savings result in lower material and labour costs, thereby reducing the levelised cost of energy for the devices. Ultimately, the developed methodology will lead to reduced manufacturing and design costs for rotor blade manufacturers and facilitate the development of more efficient and robust wind and tidal turbine devices.