Chemical reactivity of metal halide perovskite (formula ABX3) materials needs to be understood not only in the context of intrinsic stability, but also when interfaced with dissimilar materials that are required to fabricate devices. Both the B-site metal cation and halide anion (X) readily participate in redox reactions. Additionally, the A-site cation is commonly a protonated organic leading to strong acid/base chemistry. We have characterized the chemical interactions of the perovskite material, mainly CH3NH3PbI3, interfaced with the three main categories of commonly available materials: metals, oxides, and organics. We show that B-site cation redox chemistry predominates with reactive metals while underpotential deposition occurs with noble metals catalyzing a pathway to metallic Pb formation. The chemistry with metal oxides can be related to the same Faradaic reactions that give rise to gas sensing behavior meaning voltage and illumination shift the steady state concentrations of products. Weak chemical interactions between perovskite and organic materials include the permeability/solubility of iodine in the organic while strong reactions chemically alter both the perovskite and organic, even in inert atmospheres. This research identifies key reaction and degradation mechanisms to ensure high fidelity characterization of intrinsic material properties and facilitate targeted improvements to perovskite device stability.