Oxford University. Major contributions Combinatorics in Oxford in the 1970s was dominated by matroid theory, due to the influence of Dominic Welsh and Aubrey William Ingleton. Much of Seymour's early work, up to about 1980, was on matroid theory, and included three important matroid results: his D.Phil. thesis on matroids with the max-flow min-cut property (for which he won his first Fulkerson prize); a characterisation by excluded minors of the matroids representable over the three-element field; and a theorem that all regular matroids consist of graphic and cographic matroids pieced together in a simple way (which won his first Pólya prize). There were several other significant papers from this period: a paper with Welsh on the critical probabilities for bond percolation on the square lattice; a paper on edge-multicolouring of cubic graphs, which foreshadows the matching lattice theorem of László Lovász; a paper proving that all bridgeless graphs admit nowhere-zero 6-flows, a step towards Tutte's nowhere-zero 5-flow conjecture; and a paper solving the two-paths problem (also introducing the cycle double cover conjecture), which was the engine behind much of Seymour's future work. In 1980 he moved to Ohio State University, and began work with Neil Robertson. This led eventually to Seymour's most important accomplishment, the so-called "Graph Minors Project", a series of 23 papers (joint with Robertson), published over the next thirty years, with several significant results: the graph minors structure theorem, that for any fixed graph, all graphs that do not contain it as a minor can be built from graphs that are essentially of bounded genus by piecing them together at small cutsets in a tree structure; a proof of a conjecture of Wagner that in any infinite set of graphs, one of them is a minor of another (and consequently that any property of graphs that can be characterised by excluded minors can be characterised by a finite list of excluded minors); a proof of a similar conjecture of Nash-Williams that in any infinite set of graphs, one of them can be immersed in another; and polynomial-time algorithms to test if a graph contains a fixed graph as a minor, and to solve the k vertex-disjoint paths problem for all fixed k. In about 1990 Robin Thomas began to work with Robertson and Seymour. Their collaboration resulted in several important joint papers over the next ten years: a proof of a conjecture of Sachs, characterising by excluded