Technical Abstract: The soft red winter wheat Caldwell has effective resistance to leaf rust caused by Puccinia triticina. To determine the genetic basis of this resistance, Caldwell was crossed with the leaf rust susceptible spring wheat Thatcher (Tc) and the F1 plants were backcrossed to Tc to obtain backcross F2 (BCF2) families. In the seedlings the BCF2 families segregated for a single gene to P. triticina phenotype SBDG, which is likely Lr14a based on tests with BCF3 lines. In field tests the BCF2 families segregated for a single gene for adult plant resistance that was independent of the seedling resistance. BCF2 adult plants were inoculated in greenhouse tests with P. triticina phenotypes BBBD and THBJ, and resistant plants were selected and advanced by single seed descent to BCF4 lines. In greenhouse tests adult plants of some of the BCF4 lines had the same low and high infection type (IT) to different P. triticina phenotypes as the Tc isoline with Lr12. Other BCF4 plants had intermediate IT to the P. triticina phenotype THBJ. The BCF4 plants with the intermediate ITs were selected and tested as BCF5 adult plants in greenhouse and as BCF4:6 lines in field plot tests at two locations. Caldwell likely has the seedling resistance Lr14a, the adult plant gene Lr12, and an uncharacterized adult plant resistance gene that conditions an intermediate level of effective resistance in field plots.