This chapter deals with serological tools for epidemiology, starting with an ultrashort description of the immune response and of laboratory methods. It discusses titres and cutoffs, and there are some examples of seroepidemiological studies. The chapter explains some potential biases, in particular the age cohort effect. Laboratory markers are of course also important in the diagnosis of many infectious diseases, such as the pattern of liver enzymes in hepatitis or the infected red blood cells in malaria. The enzyme-linked rabbit antibodies are put in the original well, and they will bind to the human antibodies that were already stuck to the antigen. The enzyme-linked immunosorbent assay method can be used to directly get a quantitative value for the amount of antibody. If a serological test always identified the correct antibody and never any other substances in the serum, then there would be no problem.