The structural and compositional heterogeneity of biofilms poses unique problems in metal fate and transport. A starting point for quantitative understanding of biofilm-metal interactions is surface complexation theory, with roots in chemical equilibria and thermodynamics. This approach permits fitting of experimental data to a variety of mathematical models from which predictive parameters, such as K, may be extracted. Applications of more sophisticated fitting routines such as tableau (as in FITEQL) or spectra pK methods provide a better measure of the heterogeneity. There remain large theoretical and computational challenges, as there is ample evidence to suggest that the principle of additivity is problematic, owing to chemical interactions between individual sorbent phases within biofilms. And finally, the question of how bacterial metabolic activity is likely to influence metal uptake by biofilms adds yet another layer of complexity for future investigations.