A short historical review is given for the dislocation theory in the special reference to the mechanism of metal strengthening. Until the mid of 1950’s, the basic formalism of the dislocation theory has been completed. However, the further development has been confronted by various difficulties in the application of the theory to elucidate the strengthening mechanism, although some progress in problems, such as the visualization of individual dislocations and the measurement of the dislocation velocity, has been made. The present status of this research field, inclusive of recent developments, is described. The solution hardening, the work hardening, the low temperature strength and others are discussed in some details. Recent experimental treatises to examine the strengthening mechanisms are also overviewed.