  We give a brief review on the recent development of gravitational waves in
extra-dimensional theories of gravity. Studying extra-dimensional theories with
gravitational waves provides a new way to constrain extra dimensions. After a
flash look at the history of gravitational waves and a brief introduction to
several major extra-dimensional theories, we focus on the sources and spectra
of gravitational waves in extra-dimensional theories. It is shown that one can
impose limits on the size of extra dimensions and the curvature of the universe
by researching the propagations of gravitational waves and the corresponding
electromagnetic waves. Since gravitational waves can propagate throughout the
bulk, how the amplitude of gravitational waves decreases determines the number
of extra dimensions for some models. In addition, we also briefly present some
other characteristics of gravitational waves in extra-dimensional theories.
