We study in detail the structure of Grand Unified Theories derived as the
low-energy limit of orbifold four-dimensional strings. To this aim, new
techniques for building level-two symmetric orbifold theories are presented.
New classes of GUTs in the context of symmetric orbifolds are then constructed.
The method of permutation modding is further explored and SO(10) GUTs with both
45 or 54-plets are obtained. SU(5) models are also found through this
method. It is shown that, in the context of symmetric orbifold SO(10) GUTs,
only a single GUT-Higgs, either a 54 or a 45, can be present and it always
resides in an order-two untwisted sector. Very restrictive results also hold in
the case of SU(5). General properties and selection rules for string GUTs are
described. Some of these selection rules forbid the presence of some particular
GUT-Higgs couplings which are sometimes used in SUSY-GUT model building. Some
semi-realistic string GUT examples are presented and their properties briefly
discussed.Comment: 40 pages, no figures, Late