This chapter aims at giving the reader a solid basis on concepts and denitions related to pollution from the urban and highway wet weather ows. A main aspect is to show how the nature of urban runoff pollution relates to the variability in time and place of the relevant phenomena. Last but not least it is important to understand what methods can be used to quantify pollutant related aspects irrespective of the large variability that is associated with these phenomena. The large number of factors that inuence the quality of the wet weather discharges and the variability associated with these factors typically requires that a correspondingly large number of site-specic rainfall and runoff data be collected over long periods. Statistical methods for the analysis of such time series therefore have quite a different role to play compared with methods used for analysis of the variability of pollution from continuous point sources.