The title of this chapter is selected as a reminder for the reader that knowledge on and relations between transport and transformations of pollutants has a central role in environmental process engineering. It is also the case for urban drainage pollution. In Chapter 1, it was an objective to present the overall approach of urban wet weather pollution and Chapter 2 focused on a deeper understanding of its nature. Following this track, Chapter 3 is a toolbox that includes a number of fundamental physical, chemical, and biological aspects needed for engineering in terms of quality assessment and prediction within urban drainage. This chapter has therefore focused on subjects associated with both equilibrium and dynamic conditions that are relevant within physics, physicochemistry, chemistry, microbiology, and biology. It is important to realize that the quality related aspects typically take place in multiphase systems. The partitioning in terms of exchange of substances between water, species in solution, a solid phase, and a gas phase is therefore crucial.