This chapter discusses the description of nanoparticle behavior in fluids is presented and a series of examples are given to help the reader to directly test the concepts. Nanoparticles in engineered or environmental systems are considered dispersions of primary particles. In a suspension of nanoparticles, Brownian diffusion controls the long-range forces between individual nanoparticles, causing collisions that might lead to repulsion or aggregation of the particles. Aggregation of nanoparticles results in the formation and growth of clusters. The thermodynamics and kinetics of these clusters are regulated by the composition both of the fluid surrounding the particles and that of the surface. One of the major issues in the application of nanoparticles in the reservoir is the formation of clusters which can be mechanically retained within the pore-throats of the reservoir porous medium. In field applications, the mobility of the silica nanoparticles could be enhanced under the high flow rate.