Extending as they do over about 7,500 km, the Andes form the Pacific boundary of South America and represent, as it were, the backbone of this southern continent. They are part of the circumpacific mountain system, some features of which differ from those of the W-E trending orogenic zones of the earth: There are more igneous rocks present than in many parts of the Tethyan geosynclines. The tectonic structure is characterized in particular by significant normal faulting while there are fewer overthrusts or deformations caused by folding, especially in the overlying rock. The majority of the deep-focus earthquakes and megaseisms, and also most of the recent volcanic activity, are connected with this region. Prominent deep-sea trendies run parallel to the continents and to the island arcs which encircle the Pacific. The actual Andean orogen is distinguished by further dominant geological aspects: From the morphology one would think that the orogenic events were contemporaneous whereas in fact the mountain range has been formed, since the Precambrian, by temporally and spatially superimposed crustal sections. There is no such thing as a uniform Andean geosyncline, but instead there are several separate fully developed sections which were welded together during different geodynamic phases. Any attempt to give a concise general account of the problems encountered and the progress made in studying the geology of the Andes is a questionable venture. In the last twenty years so many data have been gathered in all the geoscientific disciplines in the Andean region that it has become next to impossible for a single individual to follow every aspect of the subject, let alone to write about them. It was therefore necessary right from the start to limit the subject matter and to make a selection of the new knowledge to be dealt with. For example, merely to have attempted to describe all the new stratigraphie details would have gone far beyond the scope of this book. Here, and in many other instances, it is necessary to refer the reader to the literature on the subject. In addition, the state of researdi still varies considerably from section to section of the orogen. The accuracy and the scale of the geological maps - on which the geoscientific facts are mainly based - flucuate within wide limits and in many countries the preparation of large-scale maps has only recently got under way.