Lightning is sometimes described as "a really big spark." So how "big" can a lightning flash get? The vertical extent is generally limited by the altitude of the main charge centers in the cloud (typically 6-10 km) and certainly by the cloud top, rarely more than -20 km high. The flash within large storm systems can be much longer horizontally, reaching "mesoscale" dimensions, defined here as 100 km or greater. The first hint of mesoscale lightning was gleaned from fast-scanning radar studies of the mid-1950s. Some lightning channels, in what today would be termed a leading-line trailing-stratiform mesoscale convective system (MCS), originated from the upper level of the storm's leading edge and propagated rearward down into the light precipitation zone by as much as 160 km. These reports remained mostly a curiosity for several decades until the realization that lightning routinely occurred in the stratiform region of convective storm systems from atmospheric chemistry and aviation safety studies.