99.999% copper crystals were deformed in tension using an Instron tensile tester, and the dislocation density and arrangement in the crystals were determined before, during, and after the deformations using an etch pit technique. For crystals of low initial dislocation density, it was found that a large amount of dislocation multiplication occurred prior to yielding. Experimental relationships of dislocation density versus applied stress and versus shear strain were determined. It was found that the yield stress was not related to the initial dislocation density or arrangement. The yield stress was postulated to be determined by the stress necessary to break the gliding dislocations through impurity atom barriers in the crystal.