Heat transfer or heat flow is the process whereby heat flows from regions of higher to regions of lower temperature.

The rate of heat flow depends on the mode of heat transfer and on the temperature difference across which the heat flows, as well as on the matter of the system under consideration and its geometry.

There are three types of heat flow:

• Conduction carries heat through solids, gels, or still liquids by the collision of molecules.
• Convection displaces heat by the motion of hot or cold fluids (gases or liquids). Essentially conduction to a flowing material.
• Radiation carries heat through the emission and absorption of photons (infrared electromagnetic radiation).

The electrons of a metallic solid conduct heat through the solid. Electrons also conduct electric current through conductive solids, and the thermal and electrical conductivities of most metals have about the same ratio. A good electrical conductor, such as copper, also conducts heat well.

Weatherization slows convective heat flow in buildings. Insulation slows conductive heat flow. Reflective barriers slow radiative heat flow, and often couple directly with insulation.

For more on heat transfer, see heat.