Abstract <p>This work is a systematic review of studies in the field of viscosity and rheology of drilling fluids modified with nanoparticles, based on results obtained over approximately the past 10 years. The viscosity and rheology of more than 40 different drilling fluids with silicon, aluminum, and titanium oxide nanoparticles and with diamond nanoparticles are studied. The mass concentration of nanoparticles varies from 0.06 to 4%, and their size ranges from 5 to 190 nm. Nanofluids are highly specific dispersed liquids; their properties are not described by classical theories for coarse-dispersed fluids. The presence of nanoparticles in drilling fluids, which have a much more complex structure than nanofluids, leads to extremely interesting and novel effects, which are also discussed in this paper.</p>