David A. Weitz (born October 3, 1951) is a Canadian/American physicist and Mallinckrodt Professor of Physics & Applied Physics and professor of Systems Biology at Harvard University. He is the co-director of the BASF Advanced Research Initiative at Harvard, co-director of the Harvard Kavli Institute for Bionano Science & Technology, and director of the Harvard Materials Research Science & Engineering Center. He is best known for his work in the areas of diffusing-wave spectroscopy, microrheology, microfluidics, rheology, fluid mechanics, interface and colloid science, colloid chemistry, biophysics, complex fluids, soft condensed matter physics, phase transitions, the study of glass and amorphous solids, liquid crystals, self-assembly, surface-enhanced light scattering, and diffusion-limite