  We have previously (quant-ph/9608012) shown that for quantum memories and
quantum communication, a state can be transmitted over arbitrary distances with
error $\epsilon$ provided each gate has error at most $c\epsilon$. We discuss a
similar concatenation technique which can be used with fault tolerant networks
to achieve any desired accuracy when computing with classical initial states,
provided a minimum gate accuracy can be achieved. The technique works under
realistic assumptions on operational errors. These assumptions are more general
than the stochastic error heuristic used in other work. Methods are proposed to
account for leakage errors, a problem not previously recognized.
