Event-related functional magnetic resonance imaging (ER-fMRI) methods are allowing a new spectrum of task designs to be explored with brain imaging techniques. Individual trial events can be presented rapidly, in randomly intermixed order, and the hemodynamic responses associated with individual trial events appreciated. The basis of ER-fMRI is that the hemodynamic response tracks neuronal activity on the order of seconds and, in many situations, summates over trials in a manner well predicted by a linear model--even for trials spaced as briefly as 2 sec apart. These properties are discussed, as well as certain basic characteristics of the hemodynamic response in the context of ER-fMRI.