One of the great success stories of modern molecular genetics has been the ability of biologists to isolate and characterize the genes responsible for serious inherited diseases like Huntington's disease, cystic fibrosis and myotonic dystrophy. Instrumental in these efforts has been the construction of so-called physical maps of regions of human chromosomes. A major goal of the Human Genome Project is to construct physical maps of the entire human genome. Such maps will reduce the time and expense required to isolate and study interesting chromosomal regions by many orders of magnitude. This article describes what physical maps are and how they have been used, and it outlines some of the statistical issues involved in making them.