Since 1980 a considerable amount of scientific research dealing with geochemical processes in marine sediments has been carried out. This textbook summarizes the state of the art in this field of research. The topics comprise the examination of sedimentological and physical properties of the sedimentary solid phase, of pore water and pore water constituents, organic matter as the driving force of most microbiological processes, biotic and abiotic redox reactions, carbonates and stable isotopes as proxies for paleoclimate reconstruction, metal enrichments in ferromanganese nodules and crusts as well as in hot vents and cold seeps on the seafloor. A new chapter describes properties, occurrence and formation of gas hydrates in marine sediments. The textbook ends with a chapter on model conceptions and computer models to quantify processes of early diagenesis.