ABSTRACT Liquefaction susceptibility is the measure of a soil’s ability to liquefy during earthquakes. Conventionally, liquefaction susceptibility is determined from soil properties and a subjective knowledge of the geologic conditions. There are many uncertainties in this approach because of the subjectivity in evaluating many important factors. An expert is required if the evaluation is to be performed with confidence. The application of fuzzy set theory is proposed to improve this process. The intent was to develop a method of evaluating liquefaction susceptibility that included factors usually evaluated only by an expert. The method could then be used by engineers with little experience with liquefaction. This new approach incorporates expert opinion of the importance of the subjective factors known to affect liquefaction. The engineer inputs the soil profile using linguistic descriptions which are then processed using the theory of fuzzy sets. The resulting fuzzy sets are interpreted by defining the Fuzzy Liquefaction Susceptibility Index (FLSI), a number which can be plotted and contoured to produce a map of liquefaction susceptibility. The new approach was tested by mapping the liquefaction susceptibility of Charleston, South Carolina, and comparing with other published results generated from conventional analysis. The results were quite comparable, indicating the potential for the new method to evaluate liquefaction susceptibility. The new technique is useful in creating maps of relative liquefaction hazard that can be used in municipal planning.