The layer-by-layer assembly (LBL) of hybrid polyelectrolyte/nanoparticle thin films and the structural features of thin film prepared by it will be discussed. The areas of applications, which are currently considered for LBL assemblies include photovoltaics, light-emitting diodes, high-density magnetic memory, non-linear optical switches, nanoscale interconnects, resist layer, sensors, gas separation membranes, optical filters, and others. This technique is quite simple and based on the sequential adsorption of alternating (mono)layers of oppositely charged inorganic colloids and polyelectrolytes and results in composite materials combining mechanical properties of polymers and unique physical characteristics of nanosized inorganic clusters. The nanoparticles are made prior to the assembly, and therefore, allow for the accurate control over their size and size distribution. Imbedding nanoparticles in polymer matrix makes them immobile, which eliminates phase separation even at the nanoscale. The latter significantly complicates the preparation of other composites based on nanoparticle/polymer mixtures spin-coated or painted on a substrate. As a result, LBL assemblies of nanoparticles and other composites have exceptional homogeneity, which constitutes the key requirement for many optical and electronic applications. Additionally, the fixed positions of nano-particles enables creation of intentional homogeneity in the nanoscale by taking advantage of sequential character of the deposition. Sophisticated multilayer system with gradient properties and assemblies combining nanoparticles of different materials can be made. The functionality of each layer is determined by the properties of nanoparticles involved in the LBL process and can be alternated according to the predetermined order. Examples of multilayer stacks with magnetic, electrical, and biological and mechanical properties determined by the sequence of the LBL layer will be demonstrated. The importance of the structural control will be addressed in the examples of tuning the electrical, magnetic and optical properties of the LBL films by varying the nature of interparticle contacts.