Jean-Pierre Serre, in his 1951 doctoral thesis, applies Leray's spectral sequences to fiber bundles to carry out systematic calculations of the homotopy groups of spheres, a central problem of the topology of the time that had remained largely intractable. The work represents a decisive methodological advance: it turns a relatively new technical tool into an effective computational instrument for one of the field's most resistant problems. Serre received the Fields Medal in 1954, at age 27, still, as of this entry, the youngest laureate in the history of the prize.