For engineering systems having a potential for degradation under cyclic loading ͑e.g., landslides, soil profiles subject to liquefaction, some structural systems͒, the characterization of seismic demand should include the amplitude and duration of strong shaking within the system. This article is concerned with significant-duration parameters, which are defined as the time interval across which a specified amount of energy is dissipated ͑as measured by the integral of the square of the ground acceleration or velocity͒. We develop ground-motion prediction equations for significant-duration parameters as a function of magnitude, closest site-source distance, site parameters that reflect shallow geologic conditions as well as deep basin structure, and near-source parameters. The relations are developed using a modern database and a random-effects regression procedure. We find significant duration to increase with magnitude and site-source distance ͑effects that had been identified previously͒, but also to decrease with increasing shear-wave velocity of nearsurface sediments and to increase with increasing basin depth. Parameters that principally measure the duration of body waves were also found to decrease in near-fault areas subject to forward rupture directivity, although such effects were not apparent for other duration parameters that tend to reflect the combined duration of body and surface waves.