  Gauge theories describe the interactions of the fundamental building blocks
of nature with great success. The Standard Model achieves a partial unification
of the electromagnetic and weak interactions, and it also acomodates the strong
interactions. The known quarks and leptons appear in the fundamental
representations (or singlets) of the SU(3)_cxSU(2)_L x U(1)_Y gauge symmetry.
However, larger representations (EW triplets, color sextes, etc.) could also
occur in principle. Bounds on such exotic states based on electroweak precision
tests, unitarity, perturbativity and collider searches, indicate that they
should be very heavy or may be non-existent. But why only small representations
occur in nature? Several ideas that could give some light into this problem are
discussed here, including the approach of Nielsen et al, as well as the
possible compositeness of quarks and leptons. Then, we discuss the problem
within the context of grand unified theories, where a principle of "minimal
complexity" is proposed to restrict the size of large representations, when
they are required to form unified multiplets.
