Let K be a closed subgroup of U (n) acting on the (2n + 1)-dimensional Heisenberg group H n by automorphisms. One calls (K, H n ) a Gelfand pair when the integrable K-invariant functions on H n form a commutative algebra under convolution. We prove that this is the case if and only if the coadjoint orbits for G := K H n which meet the annihilator k ⊥ of the Lie algebra k of K do so in single K-orbits. Equivalently, the representation of K on the polynomial algebra over C n is multiplicity free if and only if the moment map from C n to k * is one-to-one on K-orbits.It is also natural to conjecture that the spectrum of the quasi-regular representation of G on L 2 (G/K) corresponds precisely to the integral coadjoint orbits that meet k ⊥ . We prove that the representations occurring in the quasi-regular representation are all given by integral coadjoint orbits that meet k ⊥ . Such orbits can, however, also give rise to representations that do not appear in L 2