  A dynamical theory of traversable wormholes is detailed in spherical
symmetry. Generically a wormhole consists of a tunnel of trapped surfaces
between two mouths, defined as temporal outer trapping horizons with opposite
senses, in mutual causal contact. In static cases, the mouths coincide as the
throat of a Morris-Thorne wormhole, with surface gravity providing an invariant
measure of the radial curvature or "flaring-out". The null energy condition
must be violated at a wormhole mouth. Zeroth, first and second laws are derived
for the mouths, as for black holes. Dynamic processes involving wormholes are
reviewed, including enlargement or reduction, and interconversion with black
holes. A new area of wormhole thermodynamics is suggested.
