The thermodynamic treatment of equilibrium fluctuations in magnetic systems is discussed and applied to a variety of situations. The spectral density of the magnetic moment or flux is derived for simply-connected bodies (sphere and cylinder) and for multiply-connected systems (loop and hollow cylinder). The material is considered to be either a normal conductor, a superconductor, or a ferromagnet. The results are valid in the presence of a static magnetic field and also if the susceptibility is field dependent. No abrupt change in fluctuations occurs as a superconductor is taken through the transition temperature. The introduction of a Josephson junction or a resistive link in a superconducting loop is shown to increase the flux fluctuations markedly with a steady increase as the link is made weaker. At high frequencies the fluctuation spectrum for a superconductor approaches that for the same system in the normal state. A sum rule for the moment fluctuations is derived and related to the difference of the initial and final responses of the system to an abrupt change of external magnetic field.