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PCI Express, officially abbreviated as PCI-E or PCIe, is a computer expansion card interface format introduced by Intel in 2004. It was designed to replace the general purpose PCI expansion bus, the high end PCI-X bus and the AGP graphics card interface. Unlike previous PC expansion interfaces rather than being a bus it is structured around point to point full duplex serial links called lanes. In PCIe 1.1 (the most common version as of 2007) each lane carries 250 MB/s in each direction. PCIe 2.0 doubles this and PCIe 3.0 doubles it again.

Each slot carries one, two, four, eight, sixteen or thirty-two lanes of data between the motherboard and the card. Lane counts are written with an x prefix e.g. x1 for a single lane card and x16 for a sixteen lane card. Thirty-two lanes of 250MB/s gives a maximum transfer rate of 8 GB/s (250 MB/s x 32) in each direction for PCIe 1.1. However the largest size in common use is x16 giving a transfer rate of 4 GB/s (250 MB/s x 16) in each direction. Putting this into perspective, a single lane has nearly twice the data rate of normal PCI, a four lane slot has a comparable data rate to the fastest version of PCI-X 1.0, and an eight lane slot has a data rate comparable to the fastest version of AGP.

PCIe slots come in a variety of sizes referred to by the maximum lane count they support. A larger card will not fit in a smaller slot but a smaller card can be used in a larger slot. The number of lanes actually connected may be smaller than the number supported by the slot size. While a 16 lane card cannot be used in an 8 lane slot it can be used in a 16 lane slot with only 8 lanes connected. The number of lanes are "negotiated" during power-up or explicitly during operation. By making the lane count flexible a single standard can provide for the needs of high bandwidth cards (e.g. graphics cards, 10 gigabit ethernet cards and multiport gigabit ethernet cards) while also being economical for less demanding cards.

As well as the ordinary expansion cards for desktops and servers the PCIe electrical interface is used in a variety of other form factors including the expresscard laptop expansion card interface. PCIe is also often used to connect integrated peripherals on the motherboard