The serrated yielding exhibited by metals during tension tests at temperatures near absolute zero is the principal consideration of this paper. Stress-strain curves are presented for various metals including Armco iron, “K” Monel, and titanium at temperatures from +200 to −269 C. The variations of yield stress and ductility are shown to depend primarily on crystal structure. At −269 C, serrated yielding occurs in many metals. This is caused by adiabatic heating, and can occur independently of the deformation mechanism. A partial differential equation is derived relating load to strain hardening, strain rate, and thermal softening. At cryogenic temperatures, it is shown that thermal softening becomes so large that maximum load is exceeded, and discontinuous yielding occurs. During yielding, heating occurs, reducing the thermal-softening term, and serration ceases.