The fundamental dynamical mechanisms of the El Niño-Southern Oscillation (ENSO) phenomenon have been extensively studied since Bjerknes envisioned ocean-atmosphere interaction in the equatorial Pacific as its main cause. This chapter provides a review of the recent progress in ENSO theory based on two classes of relatively simple models: (i) the Zebiak-Cane (ZC) type models of intermediate complexity and (ii) conceptual low-order models reducible from the ZC-type models. The leading mode of ENSO variability – in reanalysis data, the ZC-type models, and comprehensive climate models – can be reduced into the simplest possible coupled oscillator known as the Recharge Oscillator (RO). Incorporating seasonality, nonlinearity, and multi-scale processes into the RO framework, allows for basic understanding of how key physical processes determine ENSO’s properties, such as its amplitude, periodicity, phase-locking, asymmetry, as well as its nonlinear rectification onto the mean state. As these key physical processes can be easily quantified in both model and reanalysis data, the RO framework can be used to assess the simulations and projections of ENSO in climate models.