Autoantibodies are diagnostic markers, useful reagents for identifying new cellular proteins and may be involved in the pathogenesis of autoimmune diseases. Therefore, autoantibody research is an essential part of clinical medicine, clinical immunology as well as molecular and cell biology. In this book, some actual aspects of the pathogenic and diagnostic relevance of autoantibodies are described. The first part deals with the pathogenic role of autoantigens and autoantibodies in the pathogenesis of autoimmune diseases. The contributors discuss the role of the cleavage of intracellular autoantigens in apoptotic and necrotic cell death, of the idiotypic network, of natural cryptic antibodies and masking inhibitors and of some autoantibody specificities, such as autoantibodies against nervous system and onconeural antigens, manganese superoxide dismutase, phospholipids and of agonist-like antireceptor antibodies as well as the possible mechanisms and potential effects of autoantibody penetration into living cells. Furthermore, the identification and possible role of new autoantigen-autoantibody systems (e.g., autoantigens of the golgi apparatus, of coiled bodies and other subnuclear domains, angiotensin AT1 receptor, transcription regulation protein ALY, hnRNP-D/AUF proteins, 70 kD subunit of human replication protein, and a novel autoantigen cross-reactive to a conformational La/SS-B epitope) have been described. Reviews and new results regarding autoantibodies as diagnostic markers in connective tissue, bullous skin, liver, gastrointestinal, endocrinological and neurological diseases are presented in the second part. Especially, the progress in the research and diagnosis of bulous skin disorders, type I diabetes, coeliac disease (tissue transglutaminase antibodies), peripheral neuropathies and diseases with autoantibodies to ion channel proteins is critically reviewed. The last topic deals with the problem of autoimmunity in tumor patients. The role and diagnostic relevance of autoantibodies, especially antibodies against CENP-F and p53, is discussed.