In the last 15 years, the estimated prevalence of autoimmune diseases increased from about 4 to 9.4%. Autoimmune co-morbidity (i.e., the association between two or more autoimmune diseases in the same patient) is present in 0.4-0.5% of the worldwide population, more frequently in patients with multiple sclerosis, rheumatoid arthritis, autoimmune thyroid disease, type 1 diabetes, inflammatory bowel disease and vitiligo. The most common associations between autoimmune diseases define two clusters: the multiple autoimmune syndromes and overlap syndromes. This review highlights the importance of multiple antibody assays in the diagnosis of multiple autoimmune and overlap syndromes and summarizes the general conditions of autoantibody detection using the model of the five W's: why, who, what, when and where. These conditions represent a new challenge for clinical and laboratory autoimmunologists, who, in a multidisciplinary arena, cooperate in investigating and treating multiple autoimmune disorders.