  We devise a method to measure the abundance of satellite halos in
gravitational lens galaxies, and apply our method to a sample of 7 lens
systems. After using Monte Carlo simulations to verify the method, we find that
substructure comprises fraction f=0.02 (median, 0.006<f<0.07 at 90% confidence)
of the mass of typical lens galaxies, in excellent agreement with predictions
of CDM simulations. We estimate a characteristic critical radius for the
satellites of 0.0001<b/arcsec<0.006 (90% confidence). For a satellite mass
function of dn/dM M^x with x=-1.8 and M_l<M<M_h, the critical radius provides
an estimate that the upper mass limit is 10^6Msun < M_h < 10^9Msun. Our
measurement confirms a generic prediction of CDM models, and may obviate the
need to invoke alternatives to CDM like warm dark matter or self-interacting
dark matter.
