In this chapter, we expound, in the framework of logic, the principles and methods of reasoning with flexible linguistic rules. First, we present natural inference with data conversion(s) according to the relation between numerical values and flexible linguistic values. Then, we present the principles and methods of reasoning with truth-degreesReasoning with truth-degrees and reasoning with degreesReasoning with degrees based on rule’s adjoint functionsAdjoint function, on the basis of rough-trueRough-true (near-trueNear-true) inference. And we discuss reasoning with degrees with data conversion(s) and approximate reasoningApproximate reasoning and computation based on reasoning with degreesReasoning with degrees. In addition, we discuss parallel reasoning with degrees.