The refractive properties and physical dimensions of optical components change with temperature, and hence also do the characteristics of optical systems. Athermalization is the principle of stabilizing the optical performance with respect to temperature, either by designing the optical elements and mounts to be mutually compensating, or by including movable corrective mechanisms. For refractive materials, two coefficients can be defined which characterize the thermooptical sensitivity, one applicable to uniform temperature changes, and the other to spatial temperature gradients. For normal optical glasses, the effects are small, but for plastics, infrared materials, and liquids, the thermal effects can be so great as to limit their usefulness. Passive athermalization is analogous to achromatism and optical systems can be designed simultaneously achromatic and athermal. The use of composite, and high expansion mounts employing plastics or fluids, gives greater control of thermo-optical effects. Active athermalization uses auxiliary power to drive compensating elements to maintain optical performance. Commercial optical design programs can be used to model and analyze thermally perturbed systems accurately.