Intrinsic paramagnetic states are ubiquitous in biological systems, and extrinsic paramagnetic probes are often introduced in studies of biomolecular structure. The measurements of electron paramagnetic resonance and magnetic susceptibility are commonly employed in investigations of biomolecular paramagnetism. How the data obtained from these measurements are used to arrive at electronic structure, and how the electronic structure and its changes relate to biologically functional processes, are the subjects of this review. Emphasis is placed upon the role of magnetic measurements in quantifying the special properties which proteins confer upon iron and copper ions.