It has been predicted by Haldane that the one-dimensional (1D) Heisenberg antiferromagnet (AF) with integer spin has a singlet ground state separated from the excited states by an energy gap. The striking susceptibility decrease as T → 0 and the absence of long range order in quasi- 1D-AF compounds with S = 1, Ni (C2H8N2)2NO2ClO4 and two other related Ni chains, and in AgVP2S6 support the existence of Haldane gap. More detailed experiments including inelastic neutron scatering in NENP reveal a splitting of the Haldane gap by planar anisotropy, in agreement with numerical calculations