  There has been recently a surge of interest in Grand Unified Theories on
orbifolds of higher dimensional spaces. In particular, the higher dimensional
doublet-triplet splitting mechanism has been of much interest. I revisit the
superstring doublet-triplet splitting mechanism in which the color triplets are
projected out by the GSO projections, while leaving the electroweak doublets in
the physical spectrum. The connection with the higher dimensional theories is
elucidated. It is shown that the doublet-triplet splitting depends crucially on
the assignment of boundary conditions in the compactified directions.
