A vacuum-tube computer, now termed a first-generation computer, is a computer that uses vacuum tubes for logic circuitry. While the history of mechanical aids to computation goes back centuries, if not millennia, the history of vacuum tube computers is confined to the middle of the 20th century. Lee De Forest invented the triode in 1906. The first example of using vacuum tubes for computation, the Atanasoff–Berry computer, was demonstrated in 1939. Vacuum-tube computers were initially one-of-a-kind designs, but commercial models were introduced in the 1950s and sold in volumes ranging from single digits to thousands of units. By the early 1960s vacuum tube computers were obsolete, superseded by second-generation transistorized computers.

Much of what we now consider part of digital computing evolved during the vacuum tube era. Initially, vacuum tube computers performed the same operations as earlier mechanical computers, only at much higher speeds. Gears and mechanical relays operate in milliseconds, whereas vacuum tubes can switch in microseconds. The first departure from what was possible prior to vacuum tubes was the incorporation of large memories that could store thousands of bits of data and randomly access them at high speeds. That, in turn, allowed the storage of machine instructions in the same memory as data—the stored program concept, a breakthrough which today is a hallmark of digital computers.

Other innovations included the use of magnetic tape to store large volumes of data in compact form (UNIVAC I) and the introduction of random access secondary storage (IBM RAMAC 305), the direct ancestor of all the hard disk drives we use today. Even computer graphics began during the vacuum tube era with the IBM 740 CRT Data Recorder and the Whirlwind light pen. Programming languages originated in the vacuum tube era, including some still used today such as Fortran & Lisp (IBM 704), Algol (Z22) and COBOL. Operating systems, such as the GM-NAA I/O, also were born in this era.

The use of cross-coupled vacuum-tube amplifiers to produce a train of pulses was described by Eccles and Jordan in 1918. This circuit became the basis of the flip-flop, a circuit with two states that became the fundamental element of electronic binary digital computers.

The Atanasoff–Berry computer, a prototype of which was first demonstrated in 1939, is now credited as the first vacuum-tube computer. However, it was not a general-purpose computer, being able to only solve a system of linear equations, and was also not very reliable.

During World War II, special-purpose vacuum-tube digital computers such as Colossus were used to break German machine (teleprinter) ciphers known as Fish. The military intelligence gathered by these systems was essential to the Allied war effort. By the end of the war 10 Mark II COLOSSI were in use at Bletchley Park; they superseded the Heath Robinson. Each COLOSSI used 1,600 vacuum tubes (Mark I) or 2,400 vacuum tubes (Mark II). The wartime codebreaking at BP was kept secret until the 1970s.

Also during the war, electro-mechanical binary computers were being developed by Konrad Zuse. The German military establishment during the war did not prioritize computer development. An experimental electronic computer circuit with around 100 tubes was developed in 1942, but destroyed in an air raid.

In the United States, work started on the ENIAC computer late in the Second World War. The machine was completed in 1945. Although one application which motivated its development was the production of firing tables for artillery, one of the first uses of ENIAC was to carry out calculations related to the development of a hydrogen bomb. ENIAC was initially programmed with plugboards and switches instead of an electronically stored program. A post-war series of lectures disclosing the design of ENIAC, and a report by John von Neumann on a foreseeable successor to ENIAC, First Draft of a Report on the EDVAC, were widely distributed and were influential in the design of post-war vacuum-tube computers.