Radio-frequency (RF) power is one of the main quantities that needs to be known accurately when designing high-frequency circuits and systems. Up to millimeter-wave frequencies, RF power traceability is established by calibrating suitable waveguide power sensors in so-called microcalorimeters, where traceability to the SI is based on DC power substitution. By using a calibrated reference sensor, the quantity RF power can be disseminated to end user devices by applying the well-known direct comparison technique. For spree-space power measurements, traceability relies on quasi-optical techniques. Comparison of both approaches in the overlapping frequency region shows good agreement within the specified measurement uncertainties. This chapter intends to give a brief overview of RF power traceability.