This special issue of Fusion Science and Technology is a comprehensive review of the main physical and technological results achieved with the Axial Symmetric Divertor EXperiment (ASDEX) Upgrade located at the Max-Planck-Institut für Plasmaphysik in Garching, Germany. It includes special chapters presenting details of machine operation and control as well as results from the main topics of scientific investigations. This introductory chapter gives a survey of the aims of ASDEX Upgrade and the evolution of the scientific goals, which were accompanied by technical modifications during the more than 10 yr of operation. It presents a short overview on hardware modifications and on the scientific program. This chapter is also a summary of major scientific results, which are then described in more detail in the following chapters.