Autoantibodies have become an important diagnostic tool for the diagnosis of connective tissue diseases (CTD) and for defining certain subgroups of these diseases. According to the reationship between clinical symptoms and the specificity of autoantibodies they can be classified into two major groups: (1) marker antibodies which are highly specific for a particular disease entity and (2) symptom specific autoantibodies associated with certain clinical expressions or subgroups. Titers of autoantibodies can vary from low to high but do usually not correlate with the status of disease activity. In order to interpret the significance of autoantibodies in CTD and to predict the course of disease it is necessary not only to identify single autoantibody specificities but to consider the combination in which they appear.