Introduction: The forces operating between colloidal particles are of utmost importance for the stability of dispersions and emulsions. Such forces also determine the adhesion between macroscopic surfaces, the adsorption of colloids onto surfaces and they are important in the initial stage of the cell adhesion process. In applied surface chemistry much effort is going into modifying the forces acting between particles in order to give the often complex technical colloidal systems the preferred properties. This has inspired a great deal of fundamental research concerned with determining which types of forces that are operating and how the strength and magnitude of these forces can be varied by addition of additives to the solution or by surface modification techniques. This review will start with a brief recapitulation about the types of forces that are important in colloidal systems and some basic properties of these forces. Next, one important method for studying these forces, the surface force technique, will be described. Finally, the type of information that can be obtained with this technique will be discussed using examples from our studies of surfactant bilayers and of adsorbed protein layers.