Book excerpt: Few processes are as important for environmental geochemistry as the interplay between the oxidation and reduction of dissolved and solid species. The knowledge of the redox conditions is most important to predict the geochemical behaviour of a great number of components, the mobilities of which are directly or indirectly controlled by redox processes. The understanding of the chemical mechanisms responsible for the establishment of measurable potentials is the major key for the evaluation and sensitive interpretation of data. This book is suitable for advanced undergraduates as well as for all scientists dealing with the measurement and interpretation of redox conditions in the natural environment.