Models for the Origin of the Knee in the Cosmic-Ray Spectrum

A sudden steepening of the cosmic-ray energy spectrum (the knee) is observed at an energy of about 3 PeV (1 PeV = $10^{15}$ eV). The experimental study of the PeV cosmic rays has intensified greatly during the last 3 years. The recent results on extensive air showers allow us to conclude that: a) the knee has an astrophysical origin; b) the `sharpness' and the fine structure of the knee rule out `Galactic Modulation' as the origin of the knee; c) most likely the knee is the result of the explosion of a single, recent, nearby supernova.