▪ Abstract The discovery of counterparts in X-ray and optical to radio wavelengths has revolutionized the study of γ-ray bursts, until recently the most enigmatic of astrophysical phenomena. We now know that γ-ray bursts are the biggest explosions in nature, caused by the ejection of ultrarelativistic matter from a powerful energy source and its subsequent collision with its environment. We have just begun to uncover a connection between supernovae and γ-ray bursts, and are finally constraining the properties of the ultimate source of γ-ray burst energy. We review here the observations that have led to this breakthrough in the field; we describe the basic theory of the fireball model and discuss the theoretical understanding that has been gained from interpreting the new wealth of data on γ-ray bursts.