Let G be a group and V a finite dimensional representation of G over an algebraically closed field k of characteristic p > 0. Let d n (V ) be the number of indecomposable summands of V ⊗n of nonzero dimension mod p. It is easy to see that there exists a limit δ(V ) := lim n→∞ d n (V ) 1/n , which is positive (and ≥ 1) iff V has an indecomposable summand of nonzero dimension mod p. We show that in this case the numberand moreover this holds for any symmetric tensor category over k of moderate growth. Furthermore, we conjecture that in fact log(c(V ) −1 ) = O(δ(V )) (which would be sharp), and prove this for p = 2, 3; in particular, for p = 2 we show that c(V ) ≥ 3 − 4 3 δ(V )+1 . The proofs are based on the characteristic p version of Deligne's theorem for symmetric tensor categories obtained in [CEO]. We also conjecture a classification of semisimple symmetric tensor categories of moderate growth which is interesting in its own right and implies the above conjecture for all p, and illustrate this conjecture by describing the semisimplification of the modular representation category of a cyclic p-group. Finally, we study the asymptotic behavior of the decomposition of V ⊗n in characteristic zero using Deligne's theorem and the Macdonald-Mehta-Opdam identity. 1.4. Conjectural classification of semisimple symmetric tensor categories and its application to Conjecture 1.2 1.5. Cyclic groups 1.6. Organization of the paper 1.7. Acknowledgements 2. Characteristic zero 2.1. Finite groups in the non-modular case 2.2. The asymptotic formula 2.3. Tensor powers of a representation of a connected reductive group 2.4. Computation of C V (s) 3. Positive characteristic 3.1. A lower bound for c n (V ) 3.2. An upper bound on dimensions of simple objects in a semisimple symmetric tensor category 3.3. Theorem 3.2 implies Theorem 1.1 3.4. Proof of Theorem 3.2 for p = 2 3.5. Proof of Theorem 3.2 for p > 2 3.6. Conjecture 1.3 implies Conjecture 1.2 4. A conjectural classification of semisimple symmetric tensor categories in positive characteristic 4.1. The conjecture 4.2. Examples 4.3. Rank 2 simple Lie algebras 4.4. Invariantless simple Lie algebras in Ver p 5. Semisimplification of the representation category of a cyclic group 5.1. Fusion rules for representations of Z/p n . 5.2. Fusion rules for the semisimplification of the representation category of Z/p n . 5.3. Categorifications of K p References