The thermally activated magnetization reversal of individual single domain particles and of particle arrays are determined experimentally. These data are used to guide the development of new models for the stability of stored information. The contributions of thermally activated magnetization reversal to noise is analyzed in systems exhibiting spin dependent transport. The spontaneous reversal of magnetization in particles by quantum tunneling, as differentiated from thermally activated reversal, is also under investigation in both ferromagnetic and antiferromagnetic nanoparticles.