Water’s natural flowing movements, such as in rivers and reservoirs, can be used in the production of electricity. Furthermore, both the tidal range (the periodic rise and fall of the sea level) and the energy contained in flow and waves can be used in the ocean energy system. Both types of energy conversion are classed as renewable energies. While the typical use of hydropower has been widespread for hundreds of years, using the ocean for energy is in its infancy. Large hydropower turbine-generator technologies are highly optimized, robust, and cost-effective designs, with peak energy conversion efficiencies of more than 93%. However, advancements for small-scale turbine-generators must reduce technology cost and enable more compact support structures and smaller physical and environmental footprints to achieve economic feasibility. The environmental performance of turbine designs continues to improve, in the form of blade shape enhancements to reduce injury to fish and aeration into turbine flow passages to improve the water quality of releases. Therefore, research and development have been focused on advanced materials and manufacturing for powertrain components, innovative hydrodynamic and mechanical concepts to reduce integrated turbine-generator size (diameter and length) and increase speed, embedded condition monitoring sensors, and powertrain design innovations that afford flexibility in selection of design objectives such as initial cost minimization, efficiency over a range of head and flow rates, and durability or ease of replacement. Ocean energy is one of the most promising resources that can be broadly split into tides, waves, tidal or marine currents, temperature gradients, and Salinity gradients. It has potential of the same order as that of the present capacity of electricity generation worldwide. The majority of ocean energy converters are fabricated from metals like steel and composite materials. Steel offers good fatigue and stress limits, while composites possess some cost and weight saving advantages over steel, but the fatigue and stress limits are not yet well understood in comparison to steel. Other wave devices are being designed to use rubber or other flexible materials as the main structural component. Composites provide many advantages for manufacturing underwater structures such as tidal turbine blades, and wave devices, which generally offer strength, fatigue-resistance, corrosion resistance, buoyancy, and cost-effectiveness. New materials are also explored to meet the needs of a wide variety of designs, many engineering and materials options, and the unpredictable environment of subsea and new ocean energy technologies. Next-generation component would drive the costs down for multiple energy conversion system solutions, including advanced controls to tune devices to extract the maximum energy from each sea state, compact high-torque, low-speed generator technologies, and corrosion- and biofouling-resistant materials and coatings. This chapter will give a brief review about state of the art of advanced materials and devices including various components for hydropower and ocean energy.