The term ‘electrojet’ is frequently used to describe the intense bands of electric curent which flow at certain times in the earth's ionosphere at an altitude of about 100 km. Two main forms of electrojet current are distinguished; an equatorial electrojet which flows during the daylight hours in a narrow band of latitudes centred on the magnetic equator, and an auroral electrojet which flows during the main phese of magnetic storms in auroral regions. Both are accompanied by abnormally large changes in the geomagnetic field components as measured at the earth's surface. This article reviews the main features of the two forms of electrojet and discusses the present theories for their origin. Ionospheric irregularities are associated with both types of electrojet, and the experimental evidence so far accumulated indicates that these are produced by plane plasma waves generated by a two-stream instability in the plasma and travelling with the acoustic velocity of about 360 m s−1.