Determination of energy and a calibrated energy scale are critical for most X-ray science. At X-ray absorption spectroscopy synchrotron beamlines, a single reference foil is often used to give a single-point transfer for energy at the active absorption K edge. Sometimes, more complex diagnostics are used. Yet, beamline or laboratory optics can result in numerous systematic errors with structure resulting from the heat load, monochromation, mechanical and thermal drift, harmonic rejection mirrors, orientation and offsets. This chapter focuses on the use of single-crystal X-ray diffraction and powder crystal X-ray diffraction standards for energy calibration across the energy range of the experiment, whether for X-ray absorption spectroscopy or X-ray diffraction.