This chapter begins with consideration of the basis of thermal expansion in gases and then move on to the solid state. This thermal expansion can be related directly to the forces between the atoms. Materials with different thermal expansion values, when used together in applications, can lead to mechanical problems. Another way to determine thermal expansion is to use the sample to prop apart the plates of a capacitor and measure the capacitance as a function of temperature. The capacitance depends directly on the distance between the plates, so this method can lead to a very accurate assessment of the dimensions of the sample as a function of temperature and, hence the thermal expansion in the capacitance dilatometer. A related way to determine thermal expansion is to use a push-rod dilatometer. In this instrument, a push-rod connected to the ferromagnetic core of a transformer pushes against the sample, due to the movement of the ferromagnetic core.