The role of ionizing radiation metrology is outlined in its major sectors of interest: medicine and radiation protection. The reliability of results is an essential requirement in radiation measurements both for medical and protection purposes. This calls primarily for measurement traceability to the primary standards, hence for appropriate instrument calibration. In ionizing radiation measurements the quantities directly measured are not necessarily the quantities of ultimate interest. Conversion procedures are then necessary to obtain a quantity from another one. The essential aspects on these procedures are presented together with the principal physical quantities, the primary standards and the calibration methods of more wide use. Just the generalities on the above subjects are described in this lecture, being it addressed to physicists and metrologists not familiar with ionizing radiation measurements.