It is well known that the reduction of nanocrytal size can result in the important modification of their properties in comparison with bulk materials because of the surface effect and size effect. Moreover, because Eu3+ ions are hypersensitive to the local microstructures, they can be used as fluorescent probes to search the local environment around Eu3+ ions. Thus, Eu3+ doped nanomaterials exhibit unique luminescent properties, which are different from the micrometer hosts. This book discusses the recent developments of photoluminescent properties of Eu3+ in different hosts.