Orientifolds: The Unique Personality Of Each Spacetime Dimension

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.