A body of literature has developed concerning “cloaking by anomalous localized resonance”. The mathematical heart of the matter involves the behavior of a divergence-form elliptic equation in the plane, ∇ · (a(x)∇u(x)) = f(x). The complex-valued coefficient has a matrix-shell-core geometry, with real part equal to 1 in the matrix and the core, and -1 in the shell; one is interested in understanding the resonant behavior of the solution as the imaginary part of a(x) decreases to zero (so that ellipticity is lost). Most analytical work in this area has relied on separation of variables, and has therefore been restricted to radial geometries. We introduce a new approach based on a pair of dual variational principles, and apply it to some non-radial examples. In our examples, as in the radial setting, the spatial location of the source f plays a crucial role in determining whether or not resonance occurs.(joint work with Robert V. Kohn, Benjamin Schweizer and Michael I. Weinstein)