The Neumann-Poincare operator appears naturally in many contexts related to interface problems. I briefly explain some important spectral properties of the NP operator. I then present two outstanding problems which can be dealt with using the NP operator: cloaking by anomalous localized resonance and analysis of stress concentration. This talk is based on two joint papers with Habib Ammari, Giulio Ciraolo, Hyundae Lee, Graeme Milton and KiHyun Yun.