This chapter focuses on beneficial aspects of the transition metals in plant metabolism, with emphasis on their structural and functional roles in various metalloenzymes. It summarizes findings concerning the function of the metal hyperaccumulation trait in plants, the biochemical, physiological, and molecular biological mechanisms of metal hyperaccumulation. Nickel was one of the last metals found to be essential for the growth of plants. Elevated concentrations of the metals mentioned earlier inhibit plant metabolism, leading to various effects depending on the metal applied, the type of affected plant, and the environmental conditions during the stress. The photosynthetic apparatus, both its primary photochemical side and its biochemical carbon-fixing part, is one of the most important sites of inhibition by many heavy metals. Plants seem to use monomeric amino acids for binding potentially toxic heavy metals. The chapter concludes with an overview of past and present attempts to use metal hyperaccumulators for cleaning up soils and mining metals.