Rudolf Halin (February 3, 1934 – November 14, 2014) was a German graph theorist, known for defining the ends of infinite graphs,[1] for Halin's grid theorem,[2][3] for extending Menger's theorem to infinite graphs,[4] and for his early research on treewidth and tree decomposition.[5] He is also the namesake of Halin graphs, a class of planar graphs constructed from trees by adding a cycle through the leaves of the given tree; earlier researchers had studied the subclass of cubic Halin graphs but Halin was the first to study this class of graphs in full generality.[6]