We present a comprehensive analysis of the convergence properties of the frame operators of Weyl-Heisenberg systems and shift-invariant systems, and relate these to the convergence of the Walnut representation. We give a deep analysis of necessary conditions and sufficient conditions for convergence of the frame operator. We show that symmetric, norm and unconditional convergence of the Walnut series are all different, but that weak and norm convergence are the same, while there are WH-systems for which the Walnut representation has none of these convergence properties. We make a detailed study of the CC-condition (a sufficient condition for WH-systems to have finite upper frame bounds) and show that (for ab rational) a uniform version of this passes to the Wexler-Raz dual. We also show that a condition of Tolimieri and Orr implies the uniform CC-condition. We obtain stronger results in the case when (g, a, b) is a WH-system and ab is rational. For example, if ab is rational, then the CC-condition becomes equivalent to the unconditional convergence of the Walnut representation-even in a more general setting. Many of the results are generalized to shift-invariant systems. We give classifications for numerous important classes of WH-systems including: ii) The WH-systems for which the frame operator extends to a bounded operator on L-p(R), for all 1 less than or equal to p less than or equal to infinity; (2) The WH-systems for which the Frame operator extends to a bounded operator on the Wiener amalgam spacer (3) The families of frames which have the same frame operator. (C) 2001 Academic Press.