We have obtained detailed observations of the onset of auroral breakup using a variety of instruments with time resolution of some tens of seconds. Rapid sequences of all-sky photographs and fast meridian scans by photometers show that breakup is usually preceded by moderate brightening, followed by fading of the auroral birghtness lasting 1 or 2 min, before the actual breakup itself. At the time of the fading there is a brief darkening of the poleward sky. Often the breakup is preceded by one or more rapid intensifications, each one preceded by local fading. Pseudobreakups may also occur without the development of a major event. A bona fide breakup may begin on the fading arc, on an adjacent arc, or in an entirely new region nearby. This optical activity is closely correlated with the development of auroral radar echoes, a feature suggesting that variations in the ionospheric and magentospheric electric and magnetic fields are responsible for the observed auroral variations. Data from the International Mag netospheric Study magnetometer network provide some indication of a correlated response by the local auroral and ionospheric currents, although this could be partly due to changes in conductivity. Riometer recordings show a slow decrease in ionospheric radio wave absorption over a period of about 10 min prior to breakup, with the largest decrease essentially to quiet time values in the region of auroral fading and subsequent breakup. The implications of these observations regarding the trigger mechanism for the expansion phase of a magnetospheric substorm are discussed.