  The study of string models including both unoriented closed strings and open
strings presents a number of new features when compared to the standard case of
models of oriented closed strings only. We review some basic features of the
construction of these models, describing in particular how gauge symmetry
breaking can be achieved in this case. We also review some peculiar properties
of the Green-Schwarz anomaly cancellation mechanism that present themselves in
lower-dimensional open-string models.(Figures not included) (Talk presented at
the Tenth National General Relativity Conference, Bardonecchia, September 1992)
