  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 ten-dimensional
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.
