Drilling fluids are designed to optimize the drilling performance and minimize the formation damage. Various compositions can be used to prepare the drilling fluids depending on the wellbore demands and environmental concerns. This chapter discusses the drilling fluid properties and their functions as well as the common additives used in the drilling operations. The recommended rheological range of different fluids is presented. In addition, the recent technologies for enhancing the drilling fluids’ efficiency (such as using nanotechnology) are discussed. The impact of several additives on the compatibility of drilling and completion fluids is presented. Finally, the interactions of drilling and completion fluid solids with the formation rocks for conventional and unconventional reservoirs are presented. Enhancing the fluids’ stability is a key factor in achieving successful drilling and completion operations. Using polymeric materials and nanoparticles as fluid additives can significantly improve the thermal stability, rheological, and filtration properties.