Abstract: Exothermic and/or endothermic chemical reactions in combination with phase change materials can produce output temperature(s) within strict tolerances without requiring expensive and complicated external equipment to generate and maintain an output temperature. Similarly, an exothermic phase change material, which generates heat as a consequence of crystallizing a supercooled liquid, can generate heat at a constant temperature, without requiring expensive and complicated external equipment, as a consequence of the liquid form of the exothermic phase change material being in equilibrium with the solid form of the exothermic phase change material. Numerous biological and chemical processes and/or diagnostic devices require a constant temperature or temperatures for set periods of time.