The Manchester Baby, also called the Small-Scale Experimental Machine (SSEM), was the first electronic stored-program computer. It was built at the University of Manchester by Frederic C. Williams, Tom Kilburn, and Geoff Tootill, and ran its first program on 21 June 1948.

The Baby was not intended to be a practical computing engine, but was instead designed as a testbed for the Williams tube, the first truly random-access memory. Described as "small and primitive" 50 years after its creation, it was the first working machine to contain all the elements essential to a modern electronic digital computer. As soon as the Baby had demonstrated the feasibility of its design, a project was initiated at the university to develop it into a full scale operational machine, the Manchester Mark 1. The Mark 1 in turn quickly became the prototype for the Ferranti Mark 1, the world's first commercially available general-purpose computer.

The Baby had a 32-bit word length and a memory of 32 words (1 kilobit, 1,024 bits). As it was designed to be the simplest possible stored-program computer, the only arithmetic operations implemented in hardware were subtraction and negation; other arithmetic operations were implemented in software. The first of three programs written for the machine calculated the highest proper divisor of 2¹⁸ (262,144), by testing every integer from 2¹⁸ downwards. This algorithm would take a long time to execute—and so prove the computer's reliability, as division was implemented by repeated subtraction of the divisor. The program consisted of 17 instructions and ran for about 52 minutes before reaching the correct answer of 131,072, after the Baby had performed about 3.5 million operations (for an effective CPU speed of about 1100 instructions per second).