CEI may stand for:
- IATA airport code for Chiang Rai International Airport
- Conferenza Episcopale Italiana
- Central European Initiative
- Cleveland Electric Illuminating Company
- Common Electrical Interface, a hardware interoperability agreement defined by the Optical Internetworking Forum
- Competitive Enterprise Institute, a libertarian think-tank
- Cycle Engineers' Institute, a screw thread pattern (see British Standard Whitworth, which includes information on topic)
- Chicago and Eastern Illinois Railroad
- Constant Energy Ignition, a proprietary point-less ignition system manufactured by Lucas Industries
- Conselho Espírita Internacional (International Spiritist Council)