MacCallum's Theorem is a result in the field of general relativity that deals with the properties of certain solutions to the Einstein field equations. It specifically addresses the conditions under which a spacetime can be described as a Kerr spacetime, which is a solution representing rotating black holes. The theorem provides criteria for identifying when a given metric can be transformed into a Kerr-like form. This is significant for understanding the behavior of rotating bodies in the universe and has implications for the study of black holes and cosmology.