The hole grating beam sampler consists of a uniform array of small circular holes in a highly polished mirror. When used as a turning flat in a high-energy laser (HEL) optical train, it produces a multiplicity of angularly separated low-power replicas of the HEL beam in both the transmitted and reflected directions. It thus provides the opportunity to perform many simultaneous diagnostic tests such as power monitoring, bore-sight alignment, jitter sensing, wavefront sensing, power-in-the-bucket measurements, and spectral purity measurements in real time during HEL operation on a noninterference basis with the intended mission. The principle of the hole grating beam sampler will be discussed in detail, and several relationships useful to the HEL systems engineer will be derived. The capabilities and limitations of this versatile HEL diagnostic tool will then be discussed in the context of several specific applications.