  We consider the problem of initiation of propagating wave in a
one-dimensional excitable fiber. In the FitzHugh-Nagumo theory, the key role is
played by ``critical nucleus'' and ``critical pulse'' solutions whose
(center-)stable manifold is the threshold surface separating initial conditions
leading to propagation and those leading to decay. We present evidence that in
cardiac excitation models, this role is played by ``critical front'' solutions.
