Chapter Twenty. Social Choice with Fuzzy Preferences Fuzzy set theory has been explicitly introduced to deal with vagueness and ambiguity. One can also use probability theory or techniques borrowed from philosophical logic. In this chapter we consider fuzzy preferences and we survey the literature on aggregation of fuzzy preferences. We restrict ourselves to “pure aggregation” theory and accordingly do not cover strategic aspects of social choice. We present Arrovian aggregation problems in a rather standard framework as well as in a very specific economic environment. We also consider a fuzzy treatment of Sens impossibility of a Paretian liberal. We distinguish two types of fuzziness: quantitative fuzziness defined via real numbers and qualitative fuzziness defined via linguistic data with a suitable order structure. We outline the thin frontier between impossibility and possibility results.