This chapter discusses the bioavailability of trace metals in freshwater environments in relationship to speciation and role of sediments in providing bioavailable metals. It considers the speciation of trace metals in freshwater, as related to the presence of various types of ligands. The chapter examines the significance of speciation with respect to bioavailability of metals to aquatic organisms, in particular to algae. It shows some examples of experimental work relating speciation and bioavailability of metals in natural waters. It focuses on elements that are of ecotoxicological significance and are often present at elevated levels in freshwater, namely copper, zinc, nickel, cobalt, cadmium, and lead. Depending on the available metal concentrations, metals are taken up by constitutive or induced membrane-bound transport proteins that differ in number and metal affinity. The acid volatile sulfide model of metal toxicity in sediments relies upon the assumption that the available metals in porewater are determining for the toxicity for benthic organisms.