Molars show a great deal of diversity in size and shape across the mammal groups.

According to the widely accepted "differentiation theory", additional cusps have arisen by budding or outgrowth from the crown, while the rivalling "concrescence theory" instead proposes that complex teeth evolved by the clustering of originally separate conical teeth.

Therian mammals (placentals and marsupials) are generally agreed to have evolved from an ancestor with tribosphenic cheek teeth, with three main cusps arranged in a triangle.

[2] Each major cusp on an upper molar is called a cone and is identified by a prefix dependent on its relative location on the tooth: proto-, para-, meta-, hypo-, and ento-.

[3] The design that is considered one of the most important characteristics of therian mammals is called a tribosphenic molar.

In tribosphenic teeth, the lower molar is divided into two regions: the three-cusped trigonid, or shearing end, and the talonid, or crushing heel.

Many paleontologists argue that it developed independently in monotremes (from australosphenidans), rather than being inherited from a common ancestor that they share with marsupials and placentals (from boreosphenidans); this idea still has some critics.

[8] Hypsodont dentition is characterized by high-crowned teeth and enamel that extends far past the gum line, which provides extra material for wear and tear.

Hypsodont molars can continue to grow throughout life, for example in some species of Arvicolinae (herbivorous rodents).

[3][11] In zalambdodont placentals, the larger inner cusp is homologous with the paracone in a tribosphenic upper molar, while the metacone is absent, reduced or fused.

Marsupial moles show the opposite condition, with the large cusp equivalent to the metacone, and the paracone absent instead.

In the lower molars, the talonid region is reduced or absent, having lost its role as a crushing basin against the protocone.

[7] Lophodont teeth are easily identified by the differentiating patterns of ridges or lophs of enamel interconnecting the cusps on the crowns.

Present in most herbivores, these patterns of lophs can be a simple, ring-like edge, as in mole rats, or a complex arrangement of series of ridges and cross-ridges, as those in odd-toed ungulates, such as equids.

[8] Lophodont molars have hard and elongated enamel ridges called lophs oriented either along or perpendicular to the dental row.

In selenodont molars (so-named after moon goddess Selene), the major cusp is elongated into crescent-shaped ridge.

[7][8] Many carnivorous mammals have enlarged and blade-like teeth especially adapted for slicing and chopping called carnassials.