The range of compounds, both paramagnetic and ferromagnetic, with potential utility as tracers for enhancing contrast in magnetic resonance imaging is very large. Fortunately, the changes in 1/T1 and 1/T2 of water protons in solution--and ultimately in tissue--induced by these agents arise from variations of only a few physical parameters. In the present work, we first consider the properties of Gd-DTPA2- in solution in some detail, including its effects on both inner and outer sphere relaxation of solvent protons. Then, by varying one or another physical parameter, we derive behavior that characterizes several classes of contrast agents with markedly different properties. These include small macrocycles, such as Gd-DOTA-; larger complexes, including oligomers and conjugates of small paramagnetic complexes with protein; magnetized particles, such as magnetite, as a function of their size; and highly anisotropic porphyrin compounds. Predictions are compared with data from the literature.