Fresnel zone plate antennas have seen extensive research in recent years. Both transmission and reflection types have been analyzed and tested, and found to give excellent performance. Parameters such as far-field patterns, gain, efficiency, frequency dependence, aberrations, scan capability, focal region properties, and off-axis performance have been examined. While most zone plates are planar, curved (spherical or paraboloidal) versions have been found to offer certain advantages, such as sharpness of focus or larger field of view. In general, millimeter-wave results are emphasized.