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Almost every machine, from a wrist watch to a giant wind turbine contains moving parts, such as gears, bearings, seals, brakes, pistons, and cams. All of these components have two surfaces which come into contact, support a load, and move with respect to each other. Sometimes it is desireable to have low friction, to save energy, or high friction, as in the case of brakes. Usually we don't want the components to wear so they are lubricated.

The study of friction, wear, lubrication and contact mechanics are all important parts of tribology. Related aspects are surface engineering (the modification of a component's surface to improve its function, for example by applying a surface coating), surface roughness, and rolling contact fatigue (where repeated contacts causes fatigue to occur).

Surfaces may look smooth, but on a microscopic scale they are rough. When two surfaces are pressed together, contact is made at the peaks of the roughness or asperities. The real area of contact can be much less than the apparent or nominal area. At the points of intimate contact, adhesion, or even local welding, can take place. If we want to slide one surface over the other then we have to apply a force to break those junctions.

The friction force is the resistance encountered when one body moves relative to another body with which it is in contact. The static friction force is how hard you have to push something to make it move, whilst the dynamic friction force is how hard you push to keep it moving. The ratio of the frictional force F to the normal force P is called the coeffiecient of friction.

Usually we want low friction (in a car engine for example) so we do not waste excessive energy getting it moving. But in same case we need high friction, in brakes for example. Friction is also important for car tyres to grip the road and between shoes and the ground for walking.

If we put a layer of oil between the surfaces then we can separate them and easily slide one over the other with reduced friction and wear. Mineral oils are the most common lubircants, but other low shear strength materials are also used; for graphite, PTFE, and soft metals like lead or gold.

The selection of the best lubricant and understanding the mechanism by which it acts to separate surfaces in a bearing or other machine componment is a major area for study in tribology.

The design of rolling bearings and gears is such that the load is supported on a small area. This leads to high stresses (about the highest stresses we find in any branch of engineering) over small areas of the components. This can cause high frcition, wear, and contact fatigue. Contact mechanics is therefore an important part of tribology.

The analysis of contact stress is frequently difficult. Simple component geometries can be analysed using hand calculations. More complex component shapes frequently require analysis by numerical methods.