In conventional chemical synthesis or chemosynthesis, reactive molecules encounter one another through random thermal motion in a liquid or vapor. In mechanosynthesis, reactive molecules are attached to molecular mechanical systems, and their encounters result from mechanical motions bringing them together in planned sequences, positions, and orientations. Mechanosynthesis can avoid unwanted reactions by keeping potential reactants apart, and can strongly favor desired reactions by holding reactants together in optimal orientations for many molecular vibration times.

Mechanosynthetic systems resemble some biological mechanisms (and primitive forms of mechanosynthesis have been performed using scanning tunneling scraping electron microscopess). So far, early molecular engineering has relied on such devices.

Broader exploitation of mechanosynthesis awaits more advanced technology for constructing molecular machine systems - including a molecular assembler or precursors thereof.

It has been suggested, notably by K. Eric Drexler, that mechanosynthesis will be fundamental to molecular manufacturing based on nanofactories capable of building macroscopic objects with atomic precision. The potential for these is disputed by some, most notably Nobel Laurate Richard Smalley, leading to a famous dispute between the two of them - see nanotechnology.

In part to resolve this and related questions about the dangers of industrial accidents and runaway events equivalent to Chernobyl and Bhopal, and the more remote issue of ecophagy, grey goo and green goo (various potential disasters arising from runaway replicators, which could be built using mechanosynthesis) the UK Royal Society and UK Royal Academy of Engineering in 2003 commissioned a study to deal with these issues and larger social and ecological implications, led by mechanical engineering professor Ann Dowling. This is widely anticipated to take a strong position on these problems and potentials - and suggest any development path to a general theory of so-called mechanosynthesis.

See also: photosynthesis, chemosynthesis