This chapter provides fundamental physico-chemical aspects of Nanoparticles (NP) at interfaces in combination with the models available to describe the adsorption process. NPs, similar to surfactant molecules, can spontaneously accumulate at the interface between two immiscible fluids providing outstanding stability to the interface. It is their small size that makes their behavior at the interface to depart from that of colloidal particles. Adsorption of NPs with amphiphilic surface decreases the interfacial tension of a fluid interface and leads to the stabilization of foams and emulsions. As a particle adsorbs, the fluid-fluid interfacial area is replaced by particle-fluid interfaces. The understanding of the dynamic adsorption of NPs at fluid interfaces is important for the selection of the optimal type of nanoparticle and suspension composition to reach the desired process performance. NPs are a promising alternative to surfactants in creating a foam, in particular in harsh environments.