A computer program that determines the stress required to move a dislocation through an array of obstacles distributed at random on a slip plane has been written. This program has been used to measure the critical shear stress of random arrays of (a) obstacles of various equal strengths, (b) obstacles with two differing strengths, (c) obstacles with a square spectrum of strengths, and (d) circular prismatic loops lying on {110} planes in a face-centered cubic crystal.