Electronics is an applied science dealing with the development of and behavior of semiconductors, electron tubes, and other devices in which the flow of electrons is controlled; it covers the behavior of electrons in gases, vacuums, conductors, and semiconductors. Its theoretical basis lies in the principles of electromagnetism and solid-state physics discovered in the late 19th and early 20th centuries. Electronics began to grow in the 1920s with the development of radio. During World War II, the US and UK concentrated resources on the invention of radar and pulse transmission methods and by 1945 they had enormous industrial capacity for producing electronic equipment. The invention of the transistor in 1948 as a small, cheap replacement for vacuum tubes led to the rapid development of computers, transistor radios, etc. Now, with the widespread use of integrated circuits, electronics plays a vital role in communications, information processing, and industry.

All electronic circuits contain both active and passive components and transducers (e.g., microphones which energy from one form to another. Sensors of light, temperature, etc., may also be present. Passive components are normally conductors and are characterized by their properties of resistance, capacitance, and inductance. One of these usually predominates, depending on the function required. Active components are electron tubes or semiconductors; they contain a source of power and control electron flow. The former may be general-purpose tubes (diodes, triodes, etc., the name depending on the number of electrodes) which rectify, amplify, or switch electric signals. Image tubes convert an electrical input into a light signal; photoelectric tubes do the reverse. Semiconductor diodes and transistors are basically sandwiches made of two different types of semiconductor, now usually perform the general functions once done by tubes, being much smaller, more robust, and generating less heat. These few basic components can build up an enormous range of circuits with different functions. Common types include: power supply (converting AC to pulsing DC and then smoothing out the pulsations); switching and timing (the logic circuits in computers are in this category); amplifiers, which increase the amplitude or power of a signal, and oscillators, used in radio and television transmitters and which generate AC signals. Demands for increased cheapness and reliability of circuits have led to the development of microelectronics.