The occurrence of soil liquefaction and ground failure during great earthquakes is one of the most crucial factors in the subsequent economic devastation and loss of lives that can result from such catastrophic events. Due to the difficulty and expense of securing and testing high-quality undisturbed samples of soils, empirical methods based on in situ tests such as the standard penetration test (SPT) or the cone penetration test (CPT) remain the dominant approaches in engineering practice for evaluating the liquefaction potential and its effect. Indeed, the simplified procedure pioneered by Seed and Idriss (1971) is perhaps the method most widely used over the last 40 years for evaluating liquefaction potential. In this procedure, developed from field observations and field and laboratory tests with a strong theoretical basis, the liquefaction potential of a soil is most often expressed as a factor of safety (FS), which is defined as the ratio of cyclic resistance ratio (CRR) over cyclic stress ratio (CSR). In the context of liquefaction assessment, CSR represents a dimensionless measure of the cyclic shear stress applied to a soil through seismic loading, and CRR represents the corresponding measure of the cyclic shear resistance of the soil. In a deterministic assessment of the liquefaction potential, liquefaction occurs if FS = 1 and does not occur if FS > 1. In many situations, it is desirable to express the liquefaction potential in terms of probability of liquefaction (PL) rather than with a factor of safety (FS). Examples of how this expression of liquefaction potential may be used can involve: (1) mapping the liquefaction potential in a district where it is easier to interpret the liquefaction potential in terms of probability rather than factor of safety; (2) post-event investigations where the conservative bias that was typically built into existing deterministic models becomes undesirable, as it may mislead the assessment; and (3) performance-based earthquake engineering, where the unbiased probability at the component level is required. Thus, there is definitely a need for estimating the probability of liquefaction.