Taste and odour problems are the major causes of consumer complaints regarding drinking water in the industrialised world. Nevertheless, as yet there are no mandatory requirements to identify the compounds that are responsible for the observed off-flavour. This thesis shows that gas chromatographic analysis with mass spectrometric and sensory detection provides an efficient tool for such investigations. The enrichment that precedes the gas chromatographic analysis can normally be based on stripping volatile or semi-volatile organic compounds. Due to the low blank level obtained with the open stripping system used in the present study, well-known odorous compounds, such as geosmin, 2- methylisoborneol (MIB) and 2,4,6-trichloroanisole (2,4,6-TCA), could be detected at levels below their threshold odour concentrations. However, this thesis also shows that compounds that are too hydrophilic to be enriched by stripping can play an important role, either as precursors of odorous compounds or as the direct cause of off-flavours. In particular, we found that natural formation of 2,4,6-trichlorophenol (2,4,6-TCP) may lead to the formation of 2,4,6-TCA in the distribution system. Furthermore, a study of a municipal treatment plant showed that short-chain carboxylic acids can be formed in slow sand filters, giving the finished water a rancid or nauseating flavour. Collectively, the results of the present study clearly show the importance of regarding the entire process from raw water supply to the consumer's tap as an integrated system. Off-flavours may originate from processes in the raw water supply, during treatment and in the distribution system.