The present work concerns interaction between biopolymers at interfaces. Examples from real systems such as saliva and bacterial surfaces as well as model systems have been studied. Using atomic force microscopy (AFM), normal forces between the surface of the filamentous bacterium Microthrix parvicella and AFM-tips have been studied, before and after treatment with specific polyaluminium compounds used at wastewater treatment plants to control the growth of the bacterium. Measurements were performed using hydrophobized and hydrophilic tips in order to explore the interactive spectrum of the bacterium and also the changes of which, if any, on treatment with polyaluminium chloride. No preferential interaction was found to any of the tips. However, the studies revealed that long-range steric repulsion dominates the interaction on approach. It was also found that the steric repulsion is reduced by addition of polyaluminium compounds, indicating a compaction of the polymer layer on the surface of the bacterium.A mucin model system, i.e. Bovine Submaxillary Mucin (BSM), adsorbed at solid surfaces has been studied using ellipsometry and quartz crystal microbalance with dissipation (QCM-D). Using the optical method ellipsometry, time resolved data on the adsorbed amount per unit area and the average layer thickness of adsorbed BSM films were determined. From QCM-D measurements the Voigt mass per unit area and viscoelastic properties of an adsorbed BSM film (including coupled water) were determined. Combining these two methods the water content of BSM adsorbed at gold, hydrophobized, and hydrophilic silica surfaces was estimated. From the results structures for BSM adsorbed on gold, hydrophobized, and hydrophilic silica were proposed. Furthermore, effects by the addition of polyaluminium chloride of different hydrolysis ratio were investigated and compared to those of AlCl3 and a ferric chloride formulation. Hence, compaction of the BSM film was initiated at concentrations as low as 0.001 mM AlCl3. At higher concentrations, related to those used at wastewater treatment plants, addition of polyaluminium chloride formulations and AlCl3 induced an initial rapid decrease in thickness followed by an increase. An increasing adsorbed mass was also observed. Combining ellipsometry and QCM-D data it was determined that aluminium species precipitate in or on top of the pre-adsorbed BSM film and the ensemble swells over time. By rinsing the precipitate layer was removed, but with some fraction remaining associated to the film. The rate of removal was slow and proceeded over a period of tens of hours. Combining the data, the swelling of the precipitate was monitored and the water content calculated. It was obvious that the increase on adsorption and decrease on depletion is mainly related to swelling. The impact of surface characteristics was also investigated and surface charge influences in terms of electrostatic interactions between the surface and the cationic aluminium compound were inferred. Friction forces between hydroxyapatite surfaces coated with human whole salivary (HWS) films were measured and the friction coefficient of the salivary films was determined. Measurements were performed using AFM-colloidal probe using two hydroxyapatite spheres. The salivary film was found to show high compressibility and mechanical resistance. Treatment with an anionic surfactant, sodium dodecyl sulphate, however induced desorption and an irreversible structural change of the film.