Lieb, Elliott H., Loss, Michael , Solovej, Jan Philip Stability of Matter in Magnetic Fields (25K, LaTeX) ABSTRACT. In the presence of arbitrarily large magnetic fields, matter composed of electrons and nuclei was known to be unstable if $\alpha$ or $Z$ is too large. Here we prove that matter {\it is stable\/} if $\alpha<0.06$ and $Z\alpha^2<0.04$.