Eutheria and Placentalia are terms used to describe major groupings within the animal class of Mammalia. The two terms are almost synonymous, although Eutheria has a slightly wider scope. One or the other term is used in scientific classification as a subclass or an infraclass.
The majority of living mammals are placental (and thus eutherian): the other two extant groups are the Monotremata and the Marsupalia; there are other groups of extinct mammals. The Placentalia are distinguished from other mammals in that the fetus is nourished during gestation via a placenta while, in general, this is not the case with other mammals. (Bandicoots are a conspicuous exception to this rule.) Placental mammals are viviparous.
Eutheria contains the placental mammals and also the nearest ancestors of placental mammals (which are known only from the fossil record). The name itself means 'true beasts'. The closest living relatives of the eutherids are within Metatheria (which was, like Eutheria, established by Huxley in 1880). This is nearly synonymous with the cohort of Marsupialia. Both taxons (Eutheria and Metatheria) are united within the supercohort of Theria.
The earliest known eutherian species is Eomaia from the Lower Cretaceous of China. Whilst undoutedly a eutherid, the hips of the animal are too narrowly built to have allowed the birth of well-developed young. This strongly suggests that a placenta could have played no role in the development of young.