Understanding the transformation of rainfall into runoff, particularly for ungauged catchments; has always been a major concern of water resources engineers and scientists.The most common approach is use of rainfall-runoff models describing the transformation.It produces the surface runoff hydrograph as a response to a rainfall hydrograph as input. In other words, the model calculates the conversion of rainfall into runoff with application of linear theories.The transformation of rainfall into runoff over a catchment is known to be very complex hydrological phenomenon, as this process is highly nonlinear, time-varying and spatially distributed. Fuzzy Logic is a technique that allows us to map an input space to output space,like the other black box modeling techniques viz. neural networks, linear systems,expert systems, differential equations etc.So, why should we use Fuzzy Logic? The basic answer is because usually it is more understandable and faster than others. This book dealt in detail the application of fuzzy logic in simulating the transformation of rainfall into runoff. Authors feel the book will be useful to research scholars, academicians, hydrologists and water resources.