The magnetocaloric effect is discussed in terms of the thermodynamic Maxwell relation and its necessary conditions. The origin of the sharp "spikes" that appear in entropy-change curves is discussed. The Maxwell relation and heat-capacity measurements are then used to determine the entropy change and are applied to the discussion of LaFe11.6Si1.4, La0.8Nd0.2Fe11.5Si1.5, and Ni43Mn46Sn11 compounds.