Radionuclide neutron sources are of fundamental importance to neutron metrology. They are the reference standards which allow quantities such as neutron fluence and neutron dose equivalent to be realized in laboratories around the world. In order to do this it is essential that the sources themselves are well characterized in terms of neutron emission rate, emission anisotropy and spectral fluence ΦE of emitted neutrons. A summary of the types of radionuclide neutron sources is given followed by detailed descriptions of the primary standardization techniques in use today, focusing on the manganese bath technique. Methods for determining the anisotropy of emission are discussed, followed by a brief description of the spectral fluence of the different source types. Finally, an overview of international comparison exercises in this area is included.