Popular since the late 1980s, especially through the work of Eric Drexler, nanotechnology has mostly involved investigation of several approaches to making essentially mechanical devices out of small numbers of atoms. One approach extrapolates chip technology, and studies placing atoms individually on solid surfaces using for example scanning probe microscopy. Another extrapolates chemical synthesis—particularly of fullerenes—and considers large molecules made for example out of carbon atoms. And another involves for example setting up fragments of DNA to try to force particular patterns of self-assembly. Most likely it will eventually be possible to have a single universal system that can manufacture almost any rigid atomic-scale structure on the basis of some kind of program. (Ribosomes in biological cells already construct arbitrary proteins from DNA sequences, but ordinary protein shapes are usually difficult to predict.) Existing work has tended to concentrate on trying to make rather elaborate components suitable for building miniature versions of familiar machines. The discoveries in this book imply however that there are much simpler components that can also be used to set up systems that have behavior with essentially any degree of sophistication. Such systems can either have the kind of chemical and mechanical character most often considered in nanotechnology, or can be primarily electronic, for example along the lines of so-called quantum-dot cellular automata. Over the next several decades applications of nanotechnology will no doubt include much higher-capacity computers, active materials of various kinds, and cellular-scale biomedical devices.