Nitrogen fixation is a vital biological process that plays a crucial role in the nitrogen cycle, contributing to the availability of this essential nutrient for plant growth. The application of nanoparticles in the context of nitrogen fixation by trees has increased major attention owing to their unique physicochemical properties. Nanoparticles (NPs) can penetrate plant tissues and interact with cellular components, influencing key processes involved in nitrogen fixation as nodulation and nitrogenase activity. NPs also act as a carrier for essential nutrients or signaling molecules, promoting nitrogen uptake. Nanoparticle size, concentration, surface charge, and chemical composition can influence their efficacy and potential toxicity. The response of trees to nanoparticles exposure depends on plant species, growth stage, and environmental conditions. Therefore, complete knowledge of the nanoparticles-tree interactions is necessary to optimize their application and minimize potential risks. This chapter provides an overview of the present state of research on the effect of nanoparticles on nitrogen fixation by trees.