This paper reviews and brings together in a comprehensive way research by the authors over the last 5 years which provides new insights into grain boundary proeutectoid cementite formation. In particular, in an Fe-1.3%C-13%Mn alloy, grain boundary cementite "allotriomorphs" do not posses the morphology approximated in the past as double spherical caps or oblate ellipsoids. Instead, three dimensional analyses show that this grain boundary cementite exhibits a dendritic morphology. Solid-state dendrites first nucleate at austenite grain edges and/or corners, and then during growth spread along austenite grain faces. Reasons why these observations were not made in many previous studies in the literature are discussed. The orientation relationship (OR) between the grain boundary cementite dendrites and at least one of the neighboring austenite grains usually corresponds to one of the previously reported (rational) cementite:austenite ORs, indicating that nucleation occurs so as to minimize the free energy of the cementite critical nucleus. Alternatively, crystallographic analyses show that there is no unique primary dendrite arm growth direction. Implications of these findings on predictive models of microstructural evolution are considered, and a new model for grain boundary proeutectoid cementite formation is proposed.