The book entitled "Rheology of Drilling Fluids" is structured into five chapters: introducing a rheology model for non-Newtonian drilling fluids, rheological considerations for drilling and enhanced oil recovery fluids, rheological properties of drilling fluids containing special additives for geothermal drilling applications, measurement of drilling fluid rheology, and modeling of thixotropic behavior and rheology of drilling muds. This book covers the main characteristics and rheological and thixotropic properties of drilling fluids. The drilling fluid is composed of a liquid phase and a solid phase. Rheological properties characterize the flow behavior of drilling fluids, including the resistance to displacement of bodies in the fluid mass. These properties allow the evaluation of the pressure and pumping energy of the drilling fluids, the washing conditions, as well as the discharge of detritus and the danger of wall erosion. To describe the behavior of rheological fluids, we use the Bingham and Ostwald de Waele models.