A review of the performance of diamond single crystals as a high heat load optical component for synchrotron x rays is given. It has been proven experimentally that the bandpass and the angular divergence of the monochromatic beam provided by a relatively thin diamond crystal used in an undulator beam are not degraded by thermal effects for a total power up to 280 W (8.7 W absorbed) at a heat flux up to 3.5 kW/mm2 (109 W/mm2 absorbed). These high heat load tests and model calculations have shown that edge-cooled diamond crystals at room temperature provide an easy and widely satisfactory solution to the heat load problems generated by undulator beams that are currently foreseen at the third-generation storage rings of the European Synchrotron Radiation Facility, the Advanced Photon Source, and the SPring-8 facilities. For this cooling geometry, diamond single crystals offer the additional advantage that beam multiplexing can be used. Currently available synthetic diamond crystals are sufficiently big for undulator beams and their crystalline perfection is adequate. Most of the crystals actually in use were prepared with their big surfaces (about 30 mm2 in size) oriented parallel to the (100) netplanes, but more recently bigger samples whose surfaces are parallel to the (111) lattice planes were obtained. Thus, diamond single crystals are superior to all other monochromator materials for undulators and for cases where a loss of a factor of 2 in flux combined with a similar gain in resolution (as compared to silicon) are compatible with the experiments.