Abstract | This paper presents an introduction to the probabilistic basis for a new set of seismic design and assessment procedures that consider explicitly nonlinear dynamic displacements. Two new seismic design and assessment guidelines make use of these developments. The new 2000 SAC steel moment-resisting seismic design and assessment guidelines prepared for FEMA and a new draft of the ISO offshore seismic guidelines use two different formats derived from this same probabilistic model.