An interesting feature of some open superstring models in D < 10 is the simultaneous presence, in the spectrum, of gauge fields and of a number of antisymmetric tensor fields. In these cases the Green-Schwarz mechanism can (and does) take a generalized form, resulting from the combined action of all the antisymmetric tensors. These novelties are illustrated referring to some simple rational models in six dimensions, and some of their implications for the low-energy effective field theory are pointed out.

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This review is devoted to open strings, and in particular to the often surprising features of their spectra. It follows and summarizes developments that took place mainly at the University of Rome "Tor Vergata" over the last decade, and centred on world-sheet aspects of the constructions now commonly referred to as "orientifolds". Our presentation aims to bridge the gap between the world-sheet analysis, that first exhibited many of the novel features of these systems, and their geometric description in terms of extended objects, D-branes and O-planes, contributed by many other colleagues, and most notably by J. Polchinski. We therefore proceed through a number of prototype examples, starting from the bosonic string and moving on to tendimensional fermionic strings and their toroidal and orbifold compactifications, in an attempt to guide the reader in a self-contained journey to the more recent developments related to the breaking of supersymmetry.