This article addresses current issues regarding serologic testing for selected STDs, particularly those for which serologic testing has been useful for diagnosis, monitoring therapy, or seroepidemiology. For each selected pathogen, the authors discuss the strengths and weaknesses of the serologic tests as diagnostic assays, as measures of disease incidence and relevance, and as prospective markers identifying risk alterations in a given population. Also, for some conditions such as syphilis, the usefulness of serology as a measure of therapy or success of disease intervention is briefly outlined.