The surface treatment of plastics as well as metals or ceramics includes a thorough surface cleaning as an essential step prior to adhesive bonding and coating processes. Besides this, surface activation of polymers is often needed because their surface free energy is too low for durable adhesion of a coating or adhesive. In this chapter various types of UV/Ozone sources with different light spectra as well as the influences of spectra and ozone concentration are investigated and compared. Also the surface wetting and adhesive bond strength as a result of UV/Ozone, atmospheric plasma, or corona treatments on thermoset, thermoplastic, and rubber materials are presented. UV/Ozone treatment was found to show an excellent cleaning performance on all kinds of materials, and especially as a very useful technique for surface functionalisation of polymers, resulting in durable adhesion both for adhesives as well as coatings. This chapter is a condensed overview of over 30 years of experiments done with UV/Ozone treatments at The Delft University of Technology.