The factors affecting the feasibility of cryogenically cooled CMOS are reviewed. This approach becomes more attractive as CMOS feature sizes shrink below 250 nm where chip performance is limited by interconnect characteristics. The impact of interconnects is demonstrated using a methodology for estimating interconnect-limited CMOS performance. The cryogenic behavior of normal and superconducting interconnects is reviewed. Cooling the best normal interconnect metals such as Al or Cu to 77 K can produce 9×lower resistivity. High-temperature superconductors can produce lower resistance at GHz clock frequencies, but would be difficult to produce on low dielectric substrates compatible with silicon technology. Performance doubling has been demonstrated for CMOS circuits operating at liquid nitrogen temperature. Comparable performance improvements may be expected down to below 100 nm if process technology is adjusted appropriately. In addition, dramatic increases in DRAM storage times result from exponential decreases in subthreshold leakage currents. Circuit reliability should increase correspondingly, apart from hot-carrier induced degradation. Thermally efficient packages and refrigerators are required for cryogenic CMOS. Microchannel heat exchangers can produce thermally efficient cryogenic packages. However, thermodynamic limits to refrigerator performance may make operation at higher cryogenic temperatures more attractive.