The conventional criteria for evaluating genetic tests include analytic validity, clinical validity, and clinical utility. Analytical validity refers to a test's ability to measure the genotype of interest accurately and reliably. Clinical validity refers to a test's ability to detect or predict the clinical disorder or phenotype associated with the genotype. Clinical utility of a test is a measure of its usefulness in the clinic and resulting changes in clinical endpoints. In addition, the utility to individuals and families of genomic information, or personal utility, should be considered. This chapter identifies methodological and data issues involved in assessing each type of validity or utility. The validity and utility of a test must be considered in a specific context, which include diagnostic testing, newborn screening, prenatal carrier screening, and family or cascade screening. Specific rare disorders addressed include cystic fibrosis, fragile X syndrome, Duchenne and Becker muscular dystrophy, spinal muscular atrophy, Huntington disease, as well as cancer associated with BRCA mutations.