ABSTRACT: This paper is concerned with the basic attributes offitzzy logic, its possible application areas in engineeringgeology and a simple slope stability application, Some uncertainties are inherent to many engineering geological applications. In the literature, two types of uncertainty such as ignorance and variability are described, Someapproaches such as fuzzy logic, probability theory etc are used io minimize thest uncertainties. The fuzzy logic,one of these techniques, is an effective tool to define some uncertainties sourced from ignorances and variabilities, Theoretically, fuzzy rules can be constructed based either an expert knowledge or on a sei of observed ormeasured data. One of the most important stage of fuzzy logic approach is the construction of membership functions. The assumption underlying fuzzy logic theory* is that the transition from membership to non-membershipis seldom a step function. Rather, there is a gradual but specifiable change from membership to non-membership. In crisp set theory, a membership Junction (pa(x)) has only Hvo values (0 and I). In this study, some memberhipJunctions defined in the literature were presented with their graphical illustrations. In order to demonstrate thestrength and use of this approach, a conventional deterministic slope stability analysis incorporated with thefuzzy logic was performed and the results were discussed A wedge failure occurred in the andésites was analyzed and the factor of safety was found as 1.24. However, it was concluded that this result did not reflect theactual condition, This was, most probably due to the uncertainties associated with the measurement of the shearstrength parameters,. Also, the stabiliiy index value was determined. According to the stability index, value thestability class of this slope is fair and the slope is prone to slide.. When performing the fuzzy logic approach, thetriangular membership functions were selected,, because, a triangular members hip function can be defined bv amaximum, a minimum and a mode value, In classical geotechnical studies, if there is no statistically significantdatabase, the use of fuzzy logic approach based on competent judgement can be accepted as an .effective way toeliminate uncertainties.. As a consequence, the fuzzy logic is attracting more and more attention in severalresearch fields because it is able to tolerate a wide range of uncertainty.