We apply a variant of the multigrade efficient congruencing method to estimate Vinogradov's integral of degree 3 for moments of order 2s, establishing strongly diagonal behaviour for 1 s 6. Consequently, the main conjecture is now known to hold for the first time in a case of degree exceeding 2. s (1 j k),(1.1)with 1 x i , y i X (1 i s). The main conjecture in Vinogradov's mean value theorem asserts that for each ε > 0, one hasan estimate that, but for the presence of the factor X ε , would be best possible (see [5, equation (7.4)]). Despite eighty years of intense investigation, such an estimate has been established only in two cases, namely the (trivial) linear case with k = 1, and the quadratic case with k = 2 in which the elementary theory of quadratic forms can be brought to bear. Our goal in this paper is the first proof of the main conjecture (1.2) in a case with k > 2.Theorem 1.1. For each ε > 0, one has J s,3 (X) ≪ X ε (X s + X 2s−6 ).The estimate for J s,3 (X) recorded in this theorem, which establishes the main conjecture in Vinogradov's mean value theorem in the cubic case k = 3, goes substantially beyond the estimates available hitherto. By means of Newton's formulae concerning the roots of polynomials, it is apparent that J s,3 (X) = s!X s + O(X s−1 ) for 1 s 3, since the solutions of (1.1) are then simply the diagonal ones with {x 1 , . . . , x s } = {y 1 , . . . , y s }. Moreover, from [6, Theorem 1.5] one has J 4,3 (X) = 4!X 4 + O(X 10/3 (log 2X) 35 ).These estimates confirm (1.2) for 1 s 4 in a particularly strong form when k = 3, though in the latter range the estimate (1.2) has been known since at