With rapidly increasing knowledge of genetic factors influencing disease, there is the potential to apply genetic tests in clinical practice. Contexts to be considered are those that could benefit the patient or individual either by diagnosis, prognosis, monitoring, screening, guiding genetics counseling, or even from “the right to know.” Some sequence variants are rare (such as the monogenic mutations causing severe disease in a few families); other variants are common polymorphisms with smaller “polygenic” effects on disease traits. The principles of gene testing are considered in this review in the context of situations where it has been claimed that such testing could be of utility. Examples are given from early life, dyslipidemias, aneurysmal disease, arrhythmias, hypertension, and dietary interactions...