Natural systems of units, systems in which the base units are fundamental physical constants, such as the gravitational constant G, the speed of light in vacuum c, the Planck constant h, the Boltzmann constant k, the Avogadro number NA, the electron charge e, the electron rest mass me. The size of the base units in natural systems of units is set by the phenomena of nature; in this they differ fundamentally from other systems of units, in which the choice of a unit is governed by the needs of measurement practice. In the idea of the German physicist M. Planck, who was the first to propose (1906) a natural system of units with the base units h, c, G, k, such a system would be independent of terrestrial conditions and valid for any time and any place in the Universe. A whole series of other natural systems of units has been proposed (those of Lewis, Hartree, Dirac and others). Natural systems of units are marked by extremely small units of length, mass and time (in Planck's system, respectively, 4.03*10-35 m, 5.42*10-8 kg and 1.34*10-43 s) and, conversely, by an enormous unit of temperature (3.63*1032 K). Natural systems of units are therefore inconvenient for practical measurements; moreover, their units can be realised with an accuracy several orders of magnitude lower than that of the base units of the International System (SI). In theoretical physics, however, the use of natural systems of units simplifies the equations and offers certain other advantages (the Hartree system of units, for example, simplifies the way the equations of quantum mechanics are written).