In this chapter, we introduce and make an all-round examination of flexible linguistic rules. First, we expound the concept and types of flexible linguistic rules; discuss the transformation and reduction of them; Then, we analyze the truth domains and logical semanticsLogical semantics of flexible linguistic rules; and analyze the mathematical essences and mathematical backgrounds of flexible linguistic rules; And then, we present a numerical modelNumerical model of a flexible linguistic rule, which provides a condition for using mathematical approach to realize reasoning and computation with flexible linguistic rules. Besides, we discuss the relations between the flexible linguistic rules and the flexible linguistic functionsFlexible linguistic function and correlations, and give a query on the practice of treating a fuzzy rule as a fuzzy relation in the traditional fuzzy set theory.