The investigation of small-scale structure in galaxy halos
and the determination of cosmological parameters benefit greatly from the combination of sensitive imaging and high angular resolution. In this talk I will show how Keck adaptive optics observations of strong gravitational lens systems (those in which multiple images of the background object are formed) can give us insight into these areas. For objects that are red, either because they are at high redshift or because they are dusty, I will demonstrate that the Keck AO imaging is of higher quality than HST observations of the same objects. I will show how the combination of lensing and high-resolution imaging has led to the detection of effectively dark substructure in galaxy halos and can be used to place competitive constraints on dark energy parameters.