The evidence for the occurrence of the biaxial nematic phase in low molecular weight thermotropic systems is examined. The optical evidence for biaxility appears to be quite unambiguous for all the eleven cases discussed here. X-ray diffraction studies on magnetically aligned samples lend support to the optical evidence, but do not provide independent proof of long range biaxial orientational order. No X-ray studies on monodomain specimens have yet been reported. A noteworthy result, intriguing from the theoretical point of view, is that the isotropic phase transforms directly to the biaxial nematic phase in a first order transition for all the eleven compounds.