Central to the application of the system of protection recommended by the International Commission on Radiological Protection (ICRP) are the physical quantity, absorbed dose, and the protection quantities, equivalent and effective dose. These protection quantities are used to set dose limits, constraints and reference levels and to test compliance in the various activities of occupational and public exposure. They are also used in the assessment of doses from medical procedures. Effective dose in particular has proved to be a very valuable quantity that allows the summation of doses from external radiation and internal doses from different radionuclides in a single risk-related value. However, while it is possible to measure external radiation exposures and estimate internal exposures down to very low levels of dose, the associated risks, principally of cancer, are much less certain. Equivalent and effective doses are calculated using simplifying assumptions to apply to reference workers and members of the public for the purpose of control of exposures. Risks to individuals or specific population groups should be calculated not using these quantities but using best scientific information. The purpose of this note is to provide a short and readily accessible account of the purpose of the ICRP dosimetric quantities, how they are calculated, how they should be used, and how they relate to risk estimation.