  We investigate the electronic structure of twin boundaries in
orthorhombically distorted high-T$_c$ materials using the quasiclassical theory
of superconductivity. At low temperatures we find a local instability to a
time-reversal symmetry breaking state at the twin boundary. This state yields
spontaneous currents along the twin boundary that are microscopically explained
by the structure of the quasiparticle bound states. We calculate the local
density of states and find a splitting in the zero-bias peak once the
spontaneous currents set in. This splitting is measurable by STM techniques and
is a unique signature of time-reversal breaking states at a twin boundary.
