The original vision of molecular size nanotechnology involved self-replication. Molecular size self-replication is plausible since it is already accomplished by cellular life. The problem is that engineered self-replication might transcend any built-in limits via mutation, out-compete organic life, and consume the biosphere. This is not a problem until nanotechnology becomes self- replicating. However, it is worth considering well before technology approaches that capacity so we have time to institutionalize protections.

The problem is discussed in Engines of Creation by Eric Drexler, one of the first books to suggest nanotechnology.