Recent advances in scanning-probe technology, in optical technology (optical "tweezers"), and in solution-phase chemistry now enable us to manipulate individual atoms and molecules. It is thus becoming possible not only to build machines at the scale of integrated electronic circuits and circuits with "wires" no thicker than an atom, but also to manipulate biological tissues and materials at the scale of individual cells, organelles, and even molecules. The implications of this technology are profound: for computer technology, for electromechanical sensors and actuators, for materials science...