  The 0th, 1st and 2nd derivatives of a ``Fermat potential'' give the three D's
of gravitational lensing: delay, deflection and distortion. Observations of
these delays, deflections and distortions for doubly and quadruply imaged
quasars give estimates of Hubble's constant, H_0. The single largest
contribution to the uncertainty in H_0 arises from the difficulty in
constraining the degree of central concentration of the lensing potential.
Fortunately, astronomers have spent a good deal of effort over the past quarter
century addressing just this question. If galaxies at z=0.5 are like nearby
galaxies, the associated systematic uncertainty in H_0 is less than 10%. The
expected lens-to-lens scatter is 20%. Results from three particularly well
constrained systems are reported.
