  I review some recent developments in our understanding of the dynamics of Z_2
orientifolds of type IIA and IIB strings and M-theory. Interesting physical
phenomena that occur in each spacetime dimension from 10 to 1 are summarized,
along with some relationships to nonperturbative effects and to M- and
F-theory. The conceptual aspects that are highlighted are: (i) orientifolds of
M-theory, (ii) exceptional gauge symmetry from open strings, (iii) certain
similarities between branes and orientifold planes. Some comments are also made
on disconnected components of orientifold moduli space.
