This chapter discusses the important use of nanoparticles is to alter wettability of reservoir rock. It describes research efforts on different uses of nanoparticles for the surfactant/polymer enhanced oil recovery (EOR) processes. One important application of such wettability alteration is to induce oil out of the oil-wet pores of tight matrix zones of naturally fractured reservoirs. A. Karimi et al. studied the wettability alteration by aqueous dispersions of zirconium oxide in carbonate cores. The chapter explores the use of silica nanoparticles to enhance the generation of CO2 hydrates and subsequent development of highly porous hydrate structure. To determine the effect of nanoparticles on reservoir rock’s wettability, a number of researchers either measured the contact angle change or carried out the Amott test. The most commonly employed nanoparticles are SiO2 particles. M. R. Nazari et al. also carried out oil-displacement experiments employing eight different nanoparticles in carbonate rock cores.