A number of applications of millimeter waves can benefit from development of low-cost, planar antenna elements, which can be combined into either phased arrays or arrays with imaging properties. Such applications range from communication, navigation and battle-field passive and active systems to remote sensing and space science instrumentation. This paper discusses the general requirements on such planar antenna elements, with specific data for three examples: (i) the Vivaldi (tapered slot) antenna, (ii) printed dipole arrays, and (iii) planar slot arrays. It is shown that such elements can be used for both prime-focus paraboloids and Cassegrain systems, with excellent aperture efficiency and beam efficiency, with the potential number of elements in the hundreds. In order to be optimally useful, planar antennas should be suitable for integration with receiver elements, i.e. mixer diodes and IF-amplifiers. It seems feasible to construct monolithic versions of these arrays in the future.