  We consider a spherical thick shell immersed in two different spherically
symmetric space-times. Using the fact that the boundaries of the thick shell
with two embedding space-times must be nonsingular hypersurfaces, we develop a
scheme to obtain the underlying equation of motion for the thick shell in
general. As a simple example, the equation of motion of a spherical dustlike
shell in vacuum is obtained. To compare our formalism with the thin shell one,
the dynamical equation of motion of the thick shell is then expanded to the
first order of its thickness. It is easily seen that the thin shell limit of
our dynamical equation is exactly that given in the literature for the dynamics
of a thin shell. It turns out that the effect of thickness is to speed up the
collapse of the shell.
