Let X be a symmetric random matrix with independent but nonidentically distributed centered Gaussian entries. We show thatwhere Sp denotes the p-Schatten class and the constants are universal. The right-hand side admits an explicit expression in terms of the variances of the matrix entries. This settles, in the case p = ∞, a conjecture of the first author, and provides a complete characterization of the class of infinite matrices with independent Gaussian entries that define bounded operators on ℓ 2 . Along the way, we obtain optimal dimension-free bounds on the moments (E X p Sp ) 1/p that are of independent interest. We develop further extensions to non-symmetric matrices and to nonasymptotic moment and norm estimates for matrices with non-Gaussian entries that arise, for example, in the study of random graphs and in applied mathematics.2000 Mathematics Subject Classification. 60B20; 46B09; 46L53; 15B52.