Environmental Context. The chemical speciation of trace metals greatly influences their biological effects. Nonetheless, no clear consensus currently exists as to when metal complexes are bioavailable, especially for field conditions. Recently, the USA EPA has incorporated the biotic ligand model (BLM) into their regulatory framework and many other countries are now examining the implications of following suit. This review examines the fundamental basis of the BLM in order to provide the reader with an understanding of its potential uses and limitations. Abstract. The biotic ligand model is a useful construct both for predicting the effects of metals to aquatic biota and for increasing our mechanistic understanding of their interactions with biological surfaces. Since biological effects due to metals are always initiated by metal bioaccumulation, the fundamental processes underlying bio-uptake are examined in this review. The model assumes that the metal of interest, its complexes, and metal bound to sensitive sites on the biological surface are in chemical equilibrium. Therefore, many of the equilibrium constants required for the model have been compiled and their methods of determination evaluated. The underlying equilibrium assumption of the BLM is also examined critically. In an attempt to identify which conditions are appropriate for its application, several documented examples of failures of the BLM are discussed. Finally, the review is concluded by identifying some important future research directions.