  A sharp maximum in the critical current $J_c$ as a function of temperature
just below the melting point of the Abrikosov flux lattice has recently been
observed in both low and high temperature superconductors. This peak effect is
strongest in twinned crystals for fields aligned with the twin planes. We
propose that this peak signals the breakdown of the collective pinning regime
and the crossover to strong pinning of single vortices on the twin boundaries.
This crossover is very sharp and can account for the steep drop of the
differential resistivity observed in experiments.
