Thermal grease is applied to the surface of a central processing unit (CPU), the "brains" of the computer, before attaching a heat sink. A heat sink helps lower the temperature of a computer by dispersing heat into the air. Thermal grease creates maximum surface-to-surface conductivity to draw heat away from the CPU, and into the heat sink. Without thermal grease imperfections in the surface of the CPU wafer and heat sink would allow air gaps, reducing the effectiveness of the heat sink leading to overheating, errors, and possible failure.

There are different types of thermal grease, each with their own level of thermal conductivity. Persons that put an extra workload on the CPU such as gamers and overclockers might be more interested in getting a highly efficient compound.

Thermal tape is the least messy and least expensive of interfaces, however it also the least effective. Thermal tape is not generally recommended.

Thermal grease pads are often included in retail CPU packages that include a heat sink and fan. The pad is commonly gray-colored with protective wrapping that should not be touched or smeared once the wrapping is removed. Thermal pads melt with heat, conforming to the surfaces of the CPU and heat sink, then stiffen when they cool. They are sufficient solutions but not considered outstanding. Before deciding on a different thermal compound read your warranty. Some manufacturers require customers to use the provided materials to keep the CPU warranty from being voided.

Silicon and zinc-based thermal grease usually comes in tube and is a white paste. It is not heat conductive itself, but does fill potential gaps between the surfaces, providing a satisfactory interface.

Ceramic-based thermal grease includes particles of materials such as aluminum oxide. Though ceramic paste is normally considered mid-grade, one popular brand of ceramic thermal grease has outperformed silver-based greases in several independent tests. This formula contains 5 sub-micron particle shapes to better fill the microscopic valleys in the CPU chip for greater thermal transfer. Maximum performance is reached after several cycles of normal use; using the system as needed then turning it off to cool. Once this period is completed, the system can be left on 24/7 if desired. The advantage of ceramic thermal grease over silver-based grease is that ceramic grease is not electrically conductive.

Silver-based thermal grease contains highly conductive metal particles. This thermal grease is excellent for conducting heat but can also conduct electricity. If accidentally applied to CPU pins or motherboard circuitry, it can cause an electrical short. Caution is advised when applying metal-based thermal grease. Used correctly, however, this type of thermal grease is generally considered superior, notwithstanding the aforementioned.

Regardless of the type of thermal grease used, follow manufacturer’s directions carefully. Do not mix different types of thermal grease. Whenever a heat sink is removed from a CPU, both surfaces must be thoroughly cleaned and new thermal grease applied before reinstalling. Rubbing alcohol can be used to safely remove most types of thermal compounds.