It is the forces between the microscopic constituents of materials which to a large extent determine the macroscopic properties. For example, it is the differences in bonding between the carbon atoms which determines the different physical properties of carbon and graphite. The same is true in colloidal systems. In colloidal systems, there are three common types of long-range interactions between particles: van der Waals forces, electrical double layer forces and steric forces. In this paper, examples as to how these forces can be modified and even manipulated will be given. To convincingly demonstrate these effects, it is necessary to measure these interaction forces. We have achieved this by using the principles of atomic force microscopy (AFM). The principle is simple, a small particle, 5-30 microm, is attached onto a small weak cantilever spring. The interaction between this particle and another particle or a surface is measured by monitoring the deflection of the spring as the two particles are moved together. In this paper, I shall give examples of direct measurements of van der Waals, electrical double layer and steric forces and show how they can be modified and how these modifications affect the properties of bulk suspensions. Similar principles are involved in the interactions of biological materials. However, nature is much cleverer than man such that many of the macromolecules on cell surfaces are able to specifically recognise only one other molecule. An example of this recognition-type interaction, namely, cholera toxin interacting with the glycolipid Gm1, will also be presented. Finally, the adhesion of cells to surfaces of different surface chemistries has been determined; this is of significance in many fields ranging from fouling of filtration membranes on the one hand to the biocompatibility of surgical implants on the other.