The challenge of biology and medicine as we move into the 21st century centers on the analysis of complex systems and networks. The brain, the immune system, development, and even signal transduction cascades represent examples of biological complexity. The tools of genomics, e.g. large-scale DNA sequencing, genome-wide genetic mapping, and large-scale DNA arrays, will offer unique opportunities to attack these problems of systems analysis. I will discuss some of these large-scale tools; their application to leading-edge problems of biology and medicine such as immunity, cancer biology, and development; and some of the challenges arising from deciphering human heredity.