The adverse environmental impact of trace metals is an issue of current concern. The increasing recognition that it is the chemical form of a metal which determines its toxicity and bioavailability presents a new challenge for analytical chemists. Techniques are required which are sufficiently sensitive and selective to enable the quantitation o f metal species at lower levels than ever before. The importance of trace metal speciation in environmental samples is reviewed in Chapter 1. The effects of inorganic complexation, organic complexation and oxidation state on metal uptake and toxicity to the biota are discussed. Special reference is made to two currently topical metals, cadmium and selenium. Cadmium is recognised as a particularly toxic metal with no beneficial effects on living organisms while selenium is a cumulative toxin which is essential at low levels. In both cases, however, toxicity is clearly dependent on the metal species present. The complexation of cadmium by Suwannee River fulvic acid is reported in Chapter 2. Metal complexation by aquatic humic materials has important consequences for metal transport and mobility in the environment. Due to the polyelectrolyte nature and heterogeneity of these naturally occuring organic ligands, characterisation of their metal binding ability is complex. The application of "3Cd NMR for this purpose is presented. The complexation of cadmium by the International Humics Substances Society fulvic acid standard is examined. The concept of selenium speciation is explored in Chapter 3. The analytical techniques currently used for selenium determination are reviewed and two novel techniques are presented. An extraction technique, termed matrix solid phase dispersion, is applied to selenium determination in milk and fish samples. A procedure is also described to enable the simultaneous determination of the two inorganic selenium species, selenite and selenate in aqueous samples. The uptake of copper by the filamentous fungus Rhizopus arrhizus is discussed in Chapter 4. Microbial biomass has the ability to sequester metal ions from aqueous solution by a variety of mechnisms. Much of the research to date relates to batch systems. In this chapter the use of lyophilised fungal cells for the removal of metal ions from aqueous solution under continous flow conditions is investigated.