In biology, placentation refers to the formation, type and structure, or arrangement of the placenta. The function of placentation is to transfer nutrients, respiratory gases, and water from maternal tissue to a growing embryo, and in some instances to remove waste from the embryo.

Placentation is best known in live-bearing mammals (theria), but also occurs in some fish, reptiles, amphibians, a diversity of invertebrates, and flowering plants. In vertebrates, placentas have evolved more than 100 times independently, with the majority of these instances occurring in squamate reptiles.

The types of placentation include:

Marginal: The ovules develop in rows near the margin on the placenta formed along the ventral suture. It occurs in monocarpellary and unilocular ovary, e.g., Leguminosae.

Parietal: The placenta is formed by the swelling up of cohering margins, and on the latter develop the ovules in rows. It occurs in bicarpellary or multicarpellary but unilocular ovary, e.g., Papaveraceae.

Axile: Here, the placentae develop from the central axis which correspond to the confluent margins of carpels. It occurs in bi-to multilocular ovary, e.g., Solanaceae, Malvaceae.

Free-central: Here, the placenta develop in the centre of the ovary as a prolongation of floral axis and the ovules are attached on this axis. It occurs in multicarpellary but unilocular ovary, e.g., Primulaceae.

Superficial or Laminar: Here, the ovules develop over the entire inner surface of the carpels. It occurs in multicarpellary ovary, e.g., Nymphaea.

Basal: The placenta develops directly on the thalamus and bears a single ovule at the base of the unilocular ovary, e.g., Compositae.