  We exploit the evolution in time of the transmission matrix following pulse
excitation of a random medium to focus radiation at a selected time delay t'
and position r. The temporal profile of a focused microwave pulse is the same
as the incident Gaussian pulse. The contrast in space at time t' of the focused
wave is determined by the participation number of transmission eigenvalues M'
and the size N' of the measured transmission matrix. The initial rise and
subsequent decay in contrast observed reflects the distribution of decay rates
of the quasi-normal modes within the sample.
