Studies were carried out on several aspects of rare earth elements (REEs), such as the theory and practice of their applications in agriculture, their geochemical behaviors in natural and agricultural ecosystems, the mechanisms for the increase of crop yield using REE-fertilizer, and their toxicology. However, limited knowledge was available for the transfer processes and the features and mechanisms of distribution and fractionation of REEs inside plants. This book covers distribution of REEs in the environment and living organisms, sources and applications of REEs, factors affecting mobility and bio-availability of REEs in soils. Moreover, bio-accumulation of REEs in plants, fractionation and transfer of REEs from soil to plants and factors affecting on fractionation of REEs in plant were also reviewed in the part I. Part II covers toxicology and cyto-physiology of REEs in plants, toxicology of REEs in soil micro-organisms, the ecotoxicological effects of REE fertilizers on soils and plants. Moreover, the toxicological effects of REE fertilizers on crops, application methods and types of REE fertilizers were also mentioned.