Most tidal turbine blades are currently made from glass or carbon fibre reinforced epoxy composites. These represent a significant part of the turbine cost, but few data are available either to validate current safety factors or to propose alternative more environmentally-friendly materials. This study, performed within the EU H2020 RealTide project, aimed to provide these data. First, a detailed investigation of the static and fatigue behavior was performed at the coupon scale, including not only those materials currently used, but also alternative recyclable thermoplastic matrix composites and natural fibre reinforced materials. Tests were performed before and after seawater saturation, in order to quantify the change in design properties with water uptake. Then a first full scale 5 meter long composite blade was designed and tested to failure. A specific test frame was built, allowing loads up to 75 tons to be applied and simulating the applied moments corresponding to service loads. Static and cyclic loads were applied and extensive instrumentation was used to detect changes in behavior, including optical fibres implanted during manufacture, acoustic emission recording, and specific instrumentation developed within the project. The results have enabled numerical simulations to be verified, and this has provided confidence in the modelling tools. These were then employed in order to propose an improved design of a lower cost blade.