Bootstrapping (from an old expression "to pull oneself up by one's bootstraps") is a technique by which a simple computer program activates a more complicated system of programs. In the start up process of a computer system, a small program such as BIOS, initializes and tests that hardware, peripherals and external memory devices are connected, then loads a program from one of them and passes control to it, thus allowing loading of larger programs, such as an operating system.

A different use of the term bootstrapping is to use a compiler to compile itself, by first writing a small part of a compiler of a new programming language in an existing language to compile more programs of the new compiler written in the new language. This solves the "chicken and egg" causality dilemma.

Bootstrapping was shortened to booting, or the process of starting up any computer, which is the most common meaning for non-technical computer users. The verb "boot" is similarly derived.

A "bootstrap" most commonly refers to the simple program itself that actually begins the initialization of the computer's operating system, like GRUB, LILO or NTLDR. Modern personal computers have the ability of using their network interface card (NIC) for bootstrapping; on IA-32 (x86) and IA-64 (Itanium) this method is implemented by PXE and Etherboot.

The computer is regarded as starting in a "blank slate" condition - either its main memory is blank, or else its content is suspect due to a prior crash. Although magnetic core memory retains its state with the power off, there would still exist the problem of loading in to it the very first program. A special start program could be very large, and given the modern affordability of read-only memory chips, could constitute the entire program to be run (as in embedded systems) but such an arrangement is inflexible. The bootstrap part would be a short simple piece of code that loads the main code.