Nuclear fusion power plants require electron cyclotron (EC) heating and current drive (H&CD) systems for plasma heating and stabilization. High-power microwave beams between 1 and 2 MW generated by gyrotrons propagate in a dedicated waveguide transmission system to reach the plasma at specific locations. Key components in this transmission system are the chemical vapor deposition diamond windows on both the torus and gyrotron sides of the reactor as they allow transmission of high-power beams while acting as confinement and/or vacuum boundaries. Diamond windows consist of a polycrystalline diamond disk integrated in a metallic housing. In the conventional configuration, there is one disk perpendicular to the beam propagation direction. A steering mechanism is then used to deploy the fixed frequency beam at different locations in the plasma. This is, for instance, the configuration used in the ITER EC H&CD system. Movable parts close to the plasma will be problematic for the lifetime of launchers in future fusion reactors like the DEMOnstration nuclear fusion reactor (DEMO) because of the higher heat loads and neutron fluxes. Therefore, one of the alternative concepts is to deploy the beams directly at the desired resonant magnetic flux surface by frequency tuning gyrotrons. In this case, diamond windows able to work in a given frequency range, like the diamond Brewster-angle window, are required. It is an elegant and compact broadband window solution with the disk inclined at the Brewster angle with respect to the beam direction. This paper shows the development and the current state of different diamond window concepts including the design, the numerical analyses, and application of standard construction nuclear codes and of a specific qualification program.