XPS was used to identify the oxidation states of titanium in nanostructured titanium films on titanium surfaces fabricated by different polishing (mechanical and chemical) and sputtering techniques. A layer of titanium dioxide was detected on both substrates, forming a protective passive film. The physiochemical and dielectric properties of this film determine the biocompatibility of titanium when used in the manufacture of medical implants. XPS was also used to investigate the bacterial modification of the surfaces of poly(ethylene terephthalate) (PET) bottles exposed to bacterial enrichment culture over a nine month period. This book chapter presents two case studies.