Thermal rearrangements of all conjugated systems containing 4n + 2 pi electron are stereospecific. This is based on preservation of orbital symmetry in the highest occupied molecular orbital. Systems containing 4n pi electrons show the opposite conrotatory mode. This is also true for rearrangements of 4n + 2 pi (where n is a whole number) electrons driven by light (photoinduced). Photoinduced rearrangements of 4n pi electron systems (where the number of electrons is divisible by 4) follow the disrotatory rule.