  A general effective field theory formalism is presented which describes the
low-energy dynamics of a 3-brane universe. In this scenario an arbitrary
four-dimensional particle theory, such as the Standard Model, is constrained to
live on the world-volume of a (3+1)-dimensional hypersurface, or ``3-brane'',
which in turn fluctuates in a higher-dimensional, gravitating spacetime. The
inclusion of chiral fermions on the 3-brane is given careful treatment. The
power-counting needed to renormalize quantum amplitudes of the effective theory
is also discussed. The effective theory has a finite domain of validity,
restricting it to processes at low enough energies that the internal structure
of the 3-brane cannot be resolved.
