This review article launches our series on genomic medicine. It provides definitions of terms commonly used in genetics, delineates the distinction between genetics and genomics, and supplies examples of the ways in which genetic information can be used in the day-to-day care of patients. The mechanisms leading to the availability of more than 100,000 proteins from only approximately 30,000 genes are described. The various common types of mutations are identified and defined, and modes of inheritance — from simple mendelian to complex to mitochondrial — are detailed.