Although the published literature abounds with studies showing contamination of aquatic environments by metals, there are very few data which actually demonstrate the biological impact of this contamination. Biological impacts such as alteration of in situ communities and demonstration of toxicity in environmental samples often occur at sites with elevated metal concentrations, but this does not prove that metals are actually responsible for these effects. Correlation is not proof of cause and effect. Metal-induced biological effects cannot usually be inferred from measured environmental concentrations because metal bio-availability can vary dramatically from site to site. Differences in metal bio-availability lead to differences in metal bio-accumulation, which in turn lead to differences in metal-induced effects. On the other hand, metal concentrations in biota are often much better indicators of potential biological impact than concentrations in the environment, because differences in metal bio-availability are automatically taken into account. Measurement of the body concentration of metals is a powerful tool for predicting metal effects, especially for non-essential and non-regulated metals. The body burden approach is more limited when applied to essential metals such as copper and zinc. Alternate methods which provide useful information on metal bio-availability, especially for copper and zinc, include measurement of metals in the overlying water during sediment toxicity tests, and sediment spiking with additional metal.