Nano computers are smaller than minicomputers and about the size of a credit card. They use semiconductor transistors and will require molecular assemblers to build. Current computer chips are inefficient and two-dimensional, while nanotechnology will allow for three-dimensional stacking of transistors with greater efficiency. Nano computers would operate similarly to current microcomputers but on a smaller physical scale, squeezing more transistors onto silicon chips. They could store and process information using chemical, biochemical, mechanical, electronic, or quantum means. Significant challenges remain in developing stable quantum nano computing and cooling the high heat produced by ultra capacitors.