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COM Express(R), a computer-on-module (COM) form factor, is a highly integrated and compact PC that can be used in a design application much like an integrated circuit component. Each COM Express Module COM integrates core CPU and memory functionality, the common I/O of a PC/AT, USB, audio, graphics (PEG), and Ethernet. All I/O signals are mapped to two high density, low profile connectors on the bottom side of the module. COM Express employs a mezzanine-based approach. The COM modules plug into a baseboard that is typically customized to the application. Over time, the COM Express mezzanine modules can be upgraded to newer, backwards-compatible versions. COM Express is commonly used in Industrial, Military/Aerospace, Gaming, Medical, Transportation, IoT, and General Computing embedded applications.
The COM Express standard was first released in 2005 by the PCI Industrial Computer Manufacturers Group (PICMG) and it sought to provide standardized module interfaces for several different target applications. It did so by defining five different module "Types" each implementing different pinout configurations and feature sets on one or two 220-pin connectors. In that way, COM Express is a standard of multiple standards. It also defined 2 module sizes (later expanded to 4) to allow additional flexibility to better serve the end application while maintaining compatibility within each module "Type."  The COM Express form factor is used in many Railway, Industrial, and Military applications. There are also efforts for a Rugged COM Express specification in the works through VITA (vita.com). COM Express is an open standard.
There are 7 different pin outs defined in the specification. The most commonly used pin outs are Type 6 and Type 10. The latest pin-out added in revision 3.0 of the COM Express specification (available from www.picmg.org) is Type 7. The Type 7 provides up to four 10 GbE interfaces and up to 32 PCIe lanes, making COM Express 3.0 appropriate for data center, server, and high-bandwidth video applications. COM Express Rev 3.0 removed legacy Type 1, Type 2, Type 3, Type 4, and Type 5, recommending that new designs should use Type 6, 7 or 10. 
Maximum available interfaces for the defined types:
|Type||Connectors||PCI Express lanes||PEG||PCI||IDE||SATA||LAN||Video||Serial||Other features||Note|
|1||AB (Single)||6||No||No||No||4||1||LVDS A & B, VGA||Legacy|
|10||AB (Single)||4||No||No||No||2||1||LVDS A only (AB (Single) channel), DDI (*2)||2 Serial COM, 1 optional CAN||USB 3.0 (*1)|
|2||AB & CD (Double)||22||Yes||Yes||1||4||1||LVDS A & B, VGA, PEG/SDVO||Legacy|
|3||AB & CD (Double)||22||Yes||Yes||No||4||3||LVDS A & B, VGA, PEG/SDVO||Legacy|
|4||AB & CD (Double)||32||Yes||No||1||4||1||LVDS A & B, VGA, PEG/SDVO||Legacy|
|5||AB & CD (Double)||32||Yes||No||No||4||3||LVDS A & B, VGA, PEG/SDVO||Legacy|
|6||AB & CD (Double)||24||Yes||No||No||4||1||LVDS A & B,VGA, 3 x DDI (*2)||2 TX/RX serial pairs with option to overlay CAN interface on 1 port||USB 3.0|
|7||AB & CD (Double)||32||Yes, for 16 lanes.||Yes||2||None||2 TX/RX serial pairs with option to overlay CAN interface on 1 port||USB 3.0, 4 x 10Gb Ethernet||Added in Rev 3.0|
(*1) Option on previously allocated SATA2 and SATA3 pins. Implementor specific. 
(*2) DDI can be adapted to DisplayPort, HDMI, DVI or SDVO in the carrier board. 
Legend: PEG - PCI Express Graphics. Legacy - not recommended for new designs.
The specification defines 4 module sizes:
- Mini: 55 × 84 mm (2.2 × 3.3 in); formerly known as nanoETXexpress
- Compact: 95 × 95 mm (3.7 × 3.7 in); formerly known as microETXexpress
- Basic: 95 × 125 mm (3.7 × 4.9 in); formerly known as ETXexpress
- Extended: 110 × 155 mm (4.3 × 6.1 in)
The COM Express specification is hosted by PICMG. It is not freely available but a paper copy may be purchased for $150USD from the PICMG website. However, the COM Express Design Guide is free to download.
- Jainandunsing, Kishan (September 9, 2004). "COM Express Tutorial" (PDF).
- "PICMG Open Modular Computing Standards". PICMG. Retrieved June 29, 2018.
- "COM Express subcommittee talks R2.1". PICMG Systems and Technology.
- "COM Express® Overview". PCIMG. Retrieved June 29, 2018.
- "COM Express Module Base Specification Revision 3.0" (PDF). PCIMG. March 31, 2017. Retrieved July 2, 2018.
- "congatec welcomes official release of the COM Express 3.0 specification". congatec. April 13, 2017. Retrieved June 29, 2018.
- PICMG website
- The Economics and use of COM Express in Embedded Applications
- COM Express Carrier Design Guide - Guidelines for designing COM Express Carrier Boards
- Purchase specification (scroll down to "PICMG COM.0 R3.0")
- COM Express Plug-and-Play Initiative
- COM Express: Scalability and flexibility for UAS sensor processing