The mainboard is the main circuit board of a microcomputer. It contains the connectors for attaching additional boards, such as the CPU, BIOS, memory, mass storage interfaces, serial and parallel ports, expansion slots, and all the controllers required to control standard peripheral devices. Collectively, all these chips that reside on the mainboard are known as the mainboard's chipset.

On most personal computers, it is possible to add memory chips directly to the mainboard in order to upgrade the computer's speed and processing capacity. You may also be able to upgrade by replacing the CPU chip, but if the old computer is too far below the level of performance desired, you may need to replace the mainboard entirely.