Molecular mapping is rapidly being extended to more species as one result of the current emphasis on genome initiatives. The choice of the population used for mapping can have important consequences on the efficiency and accessibility of the mapping information. Recombinant inbred lines offer certain advantages over other mapping populations for many species. These more or less permanent populations permit many geneticists to contribute to the mapping effort and to profit from each other's work. Recombinant inbred lines are used extensively in mouse genetics and have been used in maize to compile a detailed molecular map.