There has been a tremendous interest and investment on the rare earth (RE) and mixed metal nanoparticles (NPs), as they have found versatile applications, stretching from catalysis, sensing, biomedicine, photonics, to quantum memory and signal processing. They are also important in recent trends of energy storage application as electro catalysts in fuel cells, perovskite oxide bifunctional catalysts for batteries, electrode materials in supercapacitors, etc. However, the scarcity of REs in nature, as the name itself suggests, largely puts limitations on their use in wide-scale applications. This restriction has resulted in numerous efforts towards the generation of materials that are very effective while using the least amount of precious RE resources. There lies the opportunity of nanoscience which aims at creating small-sized materials with high functionality. For example, some of the sustainable solar energy harvesting technologies rely on a wide range of semiconductor materials that have been rationally designed at the nanoscale to increase their conversion efficiency. The RE ions can be used directly as oxides/hydroxides NPs, or can be incorporated into alloys or intermetallic compounds. The addition of RE elements as dopants into other NP structures improves their optical, magnetic, and catalytic properties. In this chapter, we have reviewed some of the exemplary literature reports on synthesis and applications of RE and mixed metal oxide NPs in diverse fields. We hope this compendium will be helpful to get an overall idea on current trend in the synthesis and application of NPs.