Johnson noise is caused by the fluctuation of charges or polarizable molecules in lossy materials. Its magnitude can best be calculated by means of statistical mechanics, which tells us that each degree of freedom must have associated with it an energy of ½ kT. This leads to the usual expressions for Johnson noise, including the fact that the available thermal noise power, from a resistor, a lossy network, a lossy dielectric, or an antenna is always kTB. In the case of a resistor, network, or dielectric, T is the temperature of the lossy material. In the case of an antenna, T is the average temperature of the environment which the antenna "sees." Consistent with the power kTB available from an antenna, there is a particular density of radiation in space which is a function of temperature and frequency. Noise figure is defined in terms of Johnson noise. Shot noise, due to the discrete nature of electron flow, is generally distinct from Johnson noise, although in some electron devices the expression for the noise in the electron flow has the same form as that for Johnson noise. When the noise in electron flow is greater or less than pure shot noise, the motions of the electrons must be in some degree correlated. In an electron stream of low noise, the random interception of a fraction of the electron flow can reduce the correlation and increase the noise. Johnson noise and shot noise have a flat frequency spectrum.