This contribution discusses the chemical stability of methylammonium (MA) halide perovskites (MAPbI(3), MAPbBr(3), and MAPbCl(3)); it considers degradation processes relevant for devices (vs T, O-2, H2O, voltage, illumination) by outlining their thermodynamic constraints and linking them to experimental observations. Thermodynamic considerations indicate that degradation under O-2 is highly favored, albeit in principle preventable by encapsulation. The same is true for H2O exposure. Intrinsic degradation is unavoidable in devices, and under real conditions, it can be thermodynamically favored, as is the case for photodecomposition. The Gibbs energies of the decomposition reactions show strong dependences on the A-cation (decomposition vs T), on the halide (vs illumination), or on both (vs O-2, H2O). The stability trends vs composition change with degradation pathway, but MAPbI(3) often appears as the most unstable. These serious stability issues appear to have no master solution, though some approaches (e.g., encapsulation, exploiting lower dimensionality, and alternative contact phases) have so far shown promise.