When two solid bodies come in contact, such as A and B in Figure 1, heat flows from the hotter body to the colder body. From experience, the temperature profile along the two bodies varies, approximately, as shown in the figure. A temperature drop is observed at the interface between the two surfaces in contact. This phenomenon is said to be a result of a thermal contact resistance existing between the contacting surfaces. Thermal contact resistance is defined as the ratio between this temperature drop and the average heat flow across the interface.
According to Fourier's law, the heat flow between the bodies is found by the relation:
where  is the heat flow,  is the thermal conductivity,  is the cross sectional area and  is the temperature gradient in the direction of flow.
From considerations of energy conservation, the heat flow between the two bodies in contact, bodies A and B, is found as:
One may observe that the heat flow is directly related to the thermal conductivities of the bodies in contact,  and , the contact area , and the thermal contact resistance, , which, as previously noted, is the inverse of the thermal conductance coefficient, .