Serological tests are, in some situations, the only method of diagnosis of some infectious diseases. The authors make a review of the literature about the most important tests used in the most common infectious diseases and also their interpretation. It is important to point out that serology should not be considered alone but compared to clinical manifestations presented by the patient. It is also necessary, in some cases, to obtain two samples of serum, to confirm if there is an increase in the antibodies' titles. The results sent from the laboratory should be interpreted based on references given by it.