Homogeneous and isotropic models of relativistic cosmology are classified according to a general scheme. There are four main groups: static, asymptotic, monotonic and oscillatory, and each group is subdivided into classes. Altogether, there are eleven classes of cosmological models. In recent years models have been proposed which contain negative pressure. Robertson 5 original scheme of classification therefore requires development, as in the present treatment, to take into account the possibility of a negative cosmic stress. As a consequence there emerges a wide variety of cosmological models, including models that are both static and stable and models that oscillate between finite values of density.