Wafer-scale integration has long been the dream of system designers. Instead of chopping a wafer into a few hundred or a few thousand chips, one would just connect the circuits on the entire wafer. What an enormous capability wafer-scale integration would offer: all those millions of circuits connected by high-speed on-chip wires. Unfortunately, the best known optical systems can provide suitably ?ne resolution only over an area much smaller than a whole wafer. There is no known way to pattern a whole wafer with transistors and wires small enough for modern circuits. Statistical defects present a ?rmer barrier to wafer-scale integration. Flaws appear regularly in integrated circuits; the larger the circuit area, the more probable there is a ?aw. If such ?aws were the result only of dust one might reduce their numbers, but ?aws are also the inevitable result of small scale. Each feature on a modern integrated circuit is carved out by only a small number of photons in the lithographic process. Each transistor gets its electrical properties from only a small number of impurity atoms in its tiny area. Inevitably, the quantized nature of light and the atomic nature of matter produce statistical variations in both the number of photons de?ning each tiny shape and the number of atoms providing the electrical behavior of tiny transistors. No known way exists to eliminate such statistical variation, nor may any be possible.