The quantum theory of electromagnetic radiation is reflected in the field of metrology by considering quantities related to the number of photons instead of conventional radiant quantities, intended in a Maxwellian frame. This paper is intended to introduce some basic methods for photon radiometry. First, classical standards are discussed for operation at optical power levels higher than some nanowatt, when detectors average on the incident radiation with a continuous quantity as an output. The way how to validate them is also discussed. Then, some fundamental concepts are introduced to bridge between classical radiometry and quantum photon radiometry. Finally, quantum photon techniques are introduced and their main characteristics are discussed, in particular the photon metrology with entangled photons.