The measurement of the magnetic susceptibility is a major tool for determining the electronic properties of paramagnetic, possibly spin-coupled transition metal centers in biomolecules and related analogs. It is a method complementary to electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), magnetic circular dichroism (MCD), nuclear magnetic resonance (NMR), and X-ray absorption spectroscopies. In exchange coupling, the magnetic state of metal ions in biological systems is a key chemical parameter, because the overlap of electron orbitals that governs exchange coupling also controls electron transfer processes. Systematic studies of molecular magnetism have something in common with many other areas of spectroscopy insofar as they serve two objectives: (1) to learn how to deduce the spectroscopic properties of a system theoretically from its structure by going back as far as possible to the first principles of quantum mechanics and (2) to draw conclusions about the geometric conformation and electronic configuration from the spectroscopic findings. This chapter delineates that the field and temperature dependence of the magnetization even of rather dilute samples can be readily measured by means of a SQUID system. From these data, the spin and the zero-field splitting of its paramagnetic centers can be deduced, as well as the strength of the exchange coupling between centers. The main difficulties in extracting this information lie in the proper removal of contributions from paramagnetic impurities and the exact determination of the metal concentration. The safest way to solve these problems is to combine the results with those obtained from complementary methods, such as EPR and Mössbauer spectroscopy.