In radio communication systems, the transmitted signal is very weak when it reaches the receiver, particularly when it has traveled over a long distance. The signal, which has shared the free-space transmission media with thousands of other radio signals, has also picked up noise of various kinds. Radio receivers must provide the sensitivity and selectivity that permit full recovery of the original intelligence signal. The radio receiver best suited to this task is known as the superheterodyne receiver. Invented in the early 1900s, the superheterodyne is used today in most electronic communication systems. This chapter reviews the basic principles of signal reception and discusses various superheterodyne circuits including direct conversion.