The chemical behavior of the actinide elements in the environment depends upon their oxidation state and chemical species. Plutonium is the major transuranic element in the environment due to releases in nuclear weapons testing. The major factors influencing environmental behavior of plutonium, including redox, compexation, colloid formation, solubility, etc. are discussed. The strong hydrolysis and low solubility of Pu(OH)4 limits the solubility concentrations of plutonium in the other, more soluble oxidation states and results in plutonium in environmental waters being mainly present in colloids. To perform valid modeling, it is necessary to have reliable knowledge on the nature of plutonium present, the oxidation state (or states) and reliable stability constants for the complexes which may be formed in these oxidation states. These problems and possible approaches to their solution are the main focus of this paper.