We have measured the energy dissipation of the quartz crystal microbalance(QCM), operating in the liquid phase, when mono- or multi-layers of bio-molecules and biofilms form on the QCM electrode (with a time resolution of ca. 1 s). Examples are taken from protein adsorption, lipid vesicle adsorption and cell adhesion studies. Our results show that even very thin (a few nm) biofilms dissipate a significant amount of energy owing to the QCM oscillation. Various mechanisms for this energy dissipation are discussed. Three main contributions to the measured increase in energy dissipation are considered. (i) A viscoelastic porous structure (the biofilm) that is strained during oscillation, (ii) trapped liquid that moves between or in and out of the pores due to the deformation of the film and (iii) the load from the bulk liquid which increases the strain of the film. These mechanisms are, in reality, not entirely separable, rather, they constitute an effective viscoelastic load. The biofilms can therefore not be considered rigidly coupled to the QCM oscillation. It is further shown theoretically that viscoelastic layers with thicknesses comparable to the biofilms studied in this work can induce energy dissipation of the same magnitude as the measured ones.