The specific binding of complementary DNA strands has been suggested as an ideal method for directing the controlled self-assembly of microscopic objects. We report the first direct measurements of such DNA-induced interactions between colloidal microspheres, as well as the first colloidal crystals assembled using them. The interactions measured with our optical tweezer method can be modeled in detail by well-known statistical physics and chemistry, boding well for their application to directed selfassembly. The microspheres’ binding dynamics, however, have a surprising power-law scaling that can significantly slow annealing and crystallization.