Among different self-assembly based techniques that have been developed for the fabrication of nanoparticle thin films, air-liquid interfacial assembly has been the most widely investigated approach based on which commercial technologies have also been developed. These developments have been inspired by the original Langmuir-Blodgett technique for organic molecules which is extended to nanoparticles to obtain nanoparticle films. This technique simply involves trapping of nanoparticles at the liquid-air interface which form a closed packed film when there are enough nanoparticles to cover the interfacial area available. As this technique is highly sensitive to the surface properties of the nanoparticles and the physicochemical properties of the subphase (the liquid used for the self-assembly), the process needs to be tailored for different nanoparticles synthesized by different methods. Thus, this technique is still being explored for further development towards more complex nanoscale arrangements and large scale applications. The common Langmuir-Blodgett trough design has a large area container (or trough) (for example, Kibron Microtrough G4 has a surface area of 930 cm 2 ) where the sub-phase (mostly water) is filled up to the top. The nanoparticles are introduced on the surface of this liquid to be trapped at the interface as shown in Figure . The surface area over the liquid is then reduced by a movable barrier so that the surface area is exactly what is needed for the nanoparticles to form a closed packed monolayer film. The movable barrier in this design is very important for using water as the subphase as this allows surface pressure regulation. Due to the high surface tension of water, introduction of nanoparticles at the air-water interface of a fixed area results in increasing surface pressure that hinders free movement of the nanoparticles to form a continuous closed packed film. Thus, the barrier allows a large surface area in the beginning for a limited number of nanoparticles to move around freely in two dimensions and are gradually packed into a smaller area by the barrier sliding over the water surface. Also, the barrier allows for different packing densities in the film, Figure (a). Further, the barrier regulates the surface pressure during the transfer of the film to a substrate by vertical dipping. An alternative simpler setup for the air-liquid interfacial assembly consists of a low surface tension liquid (e.g., ethylene glycol, diethylene glycol, etc.) contained in a container so that the liquid surface, which is of fixed area unlike the setup discussed above, acts as the interface. This method has been used extensively by Murray et al., amongst others, to obtain self-assembled films over relatively smaller areas (up to a few cm 2 ) but also for more complex arrays such as multilayer films, binary nanoparticle films and so on. [75] The low surface tension liquid facilitates easier movement of the nanoparticles at the interface in the formation of the film without requiring external arrangements for the regulation of the surface area, Figure (b). Naturally, this method offers less control over the monolayer or multilayer formation.