For this occasion of Paul Baum's 80th birthday, I would like to discuss the role played by the geometric K-homology of Baum and Douglas in the solution of the index problem for Fredholm operators (hypoelliptic, but not elliptic) in the Heisenberg calculus. This index theorem generalizes the index formula of Boutet de Monvel for Toeplitz operators to a class of pseudodifferential operators on contact manifolds. The solution of the problem depended critically on the Baum-Douglas framework of geometric K-homology.