In 1971 he conjectured that every finitely generated group is accessible. This conjecture is known as "Wall's conjecture". It motivated much progress in the understanding of splittings of groups. In 1985 Martin Dunwoody proved the conjecture for the class of finitely presented groups. The resolution of the full conjecture took until 1991 when, surprising to most mathematicians at the time, Dunwoody found a finitely generated group that is not accessible and hence the conjecture turned out to be not correct in its general formulation.