Abstract. We study the growth of the rank of elliptic curves and, more generally, Abelian varieties upon extensions of number fields.First, we show that if L/K is a finite Galois extension of number fields such that Gal(L/K) does not have an index 2 subgroup and A/K is an Abelian variety, then rk A(L) − rk A(K) can never be 1. We obtain more precise results when Gal(L/K) is of odd order, alternating, SL 2 (Fp) or PSL 2 (Fp). This implies a restriction on rk E(K(E[p])) − rk E(K(ζp)) when E/K is an elliptic curve whose mod p Galois representation is surjective. Similar results are obtained for the growth of the rank in certain non-Galois extensions.Second, we show that for every n ≥ 2 there exists an elliptic curve E over a number field K such that Q ⊗ End Q Res K/Q E contains a number field of degree 2 n . We ask whether every elliptic curve E/K has infinite rank over KQ(2), where Q(2) is the compositum of all quadratic extensions of Q. We show that if the answer is yes, then for any n ≥ 2, there exists an elliptic curve E/K admitting infinitely many quadratic twists whose rank is a positive multiple of 2 n .