The force that a magnetic field gradient can exert on a ferromagnetic material can be utilized in a number of medical applications. This force is proportional to the spatial gradient of the ambient magnetic field times the magnetic moment of the material; the magnetic moment is a function of the magnitude of the ambient field and the geometry and material used. This paper discusses the importance of each of these factors and discusses methods for calculating magnetic fields and gradients produced by permanent magnets, electromagnets, and superconducting magnets. Examples of an electromagnet and a superconducting magnet for medical applications are presented.