Rough sets and Fuzzy Logic find wider applications in various fields. Considering it, the book is written to exhibit the research work of the author which will be useful to several mathematicians and computer scientists for their technical applications. In this book, the basics of fuzzy sets, fuzzy relations, fuzzy logic, the concepts of rough sets, variable precision rough sets probabilistic rough sets and rough-fuzzy hybridizations are discussed. Further, the author describes the procedure of introducing one or two thresholds in fuzzy sets through rough approximations and also describes the applications of these concepts for indexing information system with fuzzy decision attributes. Also, this book deals with a naïve procedure of reducing the ambiguity in rough sets under crisp as well as fuzzy environment.