Placental mammals, infraclass Eutheria (meaning "true/good beast" in Greek) are the dominant group within mammals in general (a classification which also includes marsupials and monotremes), and the dominant group of terrestrial vertebrates. This has been the case since the extinction of the non-avian dinosaurs 65.5 million years ago. Although placental mammals consist of fewer species (about 4,900 total) than reptiles (8,200 species), amphibians (6,100 species), or birds (10,000 species), placental mammals are dominant in that they are the most numerous, occupy the most niches, are the largest, and a placental mammal almost always occupies the highest positions in the terrestrial food chain, Australia being the primary counterexample.

Placental mammals consist of 20 orders contained within four superorders: Xenarthra (an early placental mammal breakaway family includes armadillos, sloths, and anteaters); Laurasiatheria (including the bulk of mammal species), Afrotheria (smaller group of Africa-originating animals including tenrecs, aardvarks, hyraxes, golden moles, elephant shrews, elephants and manatees); and Euarchontoglires (a sister group of Laurasiatheria that includes rabbits, hares, rodents, primates, treeshrews, and colugos). These superorder arrangements are based on varying levels of support from molecular genetics and fossil evidence, though some are quite controversy, substantially contradicting earlier classifications based on morphology.

胎盘哺乳动物的四个最大的目是Chiroptera（蝙蝠）、Rodentia（小鼠、大鼠）、Carnivora（狗、猫、熊、其他胎盘食肉动物）和Cetartiodactyla（所有偶蹄类动物，如猪和水牛，以及鲸类，包括鲸鱼和海豚）。 尽管物种数量众多，前两个目中总的形态变化有点有限，但后两个目中却很大。 很难想象猪和羊与鲸和海豚属于同一个目，但这是真的。 这些群体在大约6000万年前，在胎盘哺乳动物多样化的第一个大浪中分裂出来。

今天，按个体计算，所有胎盘哺乳动物中的大多数是人类、我们的宠物、牲畜和适应与我们生活在一起的动物，特别是老鼠。 在我们在世界舞台上短暂的20万年期间，人类从根本上重塑了胎盘哺乳动物的生物多样性。 当人类传播到世界各地，带来长矛和斧头等狩猎工具时，数百甚至数千种胎盘哺乳动物灭绝了。 结果，许多过去令人印象深刻的胎盘哺乳动物巨型动物，如剑齿猫和猛犸，现在完全灭绝了。 由于生境破坏和其他因素，无数胎盘哺乳动物继续灭绝了。