Irreversible features, which dominate the magnetic data in high temperature superconductors, are reviewed. Special attention is given to two phenomena: (i) The temperature and field dependence of the relaxation rate and its implication on measurement of critical fields. (ii) The field dependence of the magnetization which exhibits scaling features below the irreversibility line. The discrepancy in the barrier heights from magnetic relaxation and from resistivity measurements is discussed. © 1989.