Development ofthe ability to design protein molecules will open a path to the fabrication of devices to complex atomic specifications, thus sidestepping obstacles facing. conventional microtechnology. This path will involve construction of molecular machinery able to position;reactive groups to atomic precision. It could lead to great advances in computational devices and in the ability to manipulate biological materials. The existence of this path has implications for the present. Feynman's .1959 talk entitled "There's Plenty of Room at the Bottom" (1) discussed microtechnology as a 'frontier to be pushed back, like the frontiers of high pressure, low temperature, or high vacuum. He suggested that ordinary machines could build smaller machines that could build still smaller machines, working step by step down toward the molecular level; he also suggested using particle beams to. define two-dimensional patterns. Present microtechnology (exemplified by integrated circuits) has realized some ofthe potential outlined by Feynman by following the same basic~approach: working down from the macroscopic level to the microscopic.Present microtechnology (2) handles statistical populations of atoms. As the devices shrink, the atomic graininess of matter increasingly creates irregularities and imperfections, so long as atoms are handled in bulk, rather than individually. Indeed, such miniaturization of bulk processes seems unable to reach the ultimate level of microtechnology-the structuring of matter to complex atomic specifications. In this paper, I will outline a path to this goal, a general molecular engineering technology. The existence of this path will be shown to have implications for the present.Although the capabilities described may not prove necessary to the achievement of any particular objective, they will prove sufficient for the achievement ofan extraordinary range of objectives in which the ;structuringand analysis ofmatter are con--cerned. The claim that devices can be built to complex atomic specifications should not, however, be construed to deny the inevitability of a finite error rate arising from thermodynamic effects (and radiation damage). Such errors can be minimized through.the use of free energy in error-correcting 'procedures (including rejection offaulty components before device assembly); the effects of errors can be minimized through fault-tolerant design, as in macroscopic engineering.The emphasis on devices that have general capabilities should be taken in the spirit of early work on the theoretical capabilities ofcomputers, which did not attempt to predict such practical embodiments as specialized or distributed computation systems. The present argument, however, will -proceed from step to step by close analogies between the proposed steps and past developments in-nature. and technology, rather than by mathematical proof. We commonly accept the feasibility of new devices without formal proof, where analogies to existing systems are close enough: consider the feasibility of making ...