Nanoparticulate TiO₂ is the prototypical materials system for research on disperse systems for solar energy conversion and photocatalysis. It is therefore surprising how many open issues and controversial points have remained in trying to reach reliable predictions for newly designed TiO₂ based materials and their functional properties. Our still limited knowledge of nanoscale effects and the chemistry and photochemistry thereof complicate the systematic improvement of TiO₂ particle based materials.^1,2  At the same time, it still encourages more research on TiO₂ based model systems. However, the impact of factors underpinning the photocatalytic activity of a given semiconductor material is complex and convoluted. One current idea in the scientific literature is that there are too many intermingled factors that are associated with the photophysics, surface chemistry, as well as with the defect properties, to arrive at a scientifically sound concept for the rational design and optimization of photocatalyst materials.^3–7  It seems that this idea increasingly gains ground. Thus, the question – being relevant to the entire field of catalysis research – is whether conclusions from photoexcitation studies, which have been carried out on model materials of reduced compositional and structural complexity, can ever be general enough to be linked to working catalysts in a relevant way (Figure 8.1).