On the Equilibrium of Planes (Ancient Greek: Περὶ ἐπιπέδων ἱσορροπιῶν, romanized: perí epipédōn isorropiôn) is a treatise by Archimedes in two books.

[1] According to Pappus of Alexandria, Archimedes' work on levers and his understanding of mechanical advantage caused him to say: "Give me a place to stand on, and I will move the Earth" (Ancient Greek: δός μοί ποῦ στῶ καὶ κινῶ τὴν γῆν, romanized: dṓs moi poû stṓ kaí kinô tḗn gên), though other ancient testimonia are ambiguous regarding the context of the saying.

[5] The earlier Mechanical Problems, once attributed to Aristotle but most likely written by one of his successors, contains a loose proof of the law of the lever without employing the concept of centre of gravity.

[7] On the Equilibrium of Planes I, which contains seven postulates and fifteen propositions, uses the centre of gravity for both commensurable and incommensurable magnitudes to justify the law of the lever, though some argue not satisfactorily.

[8][10] In the Middle Ages, some Arabic authors were familiar with and extended Archimedes' work on balances and centre of gravity; in the Latin West, however, these ideas were virtually unknown except for a handful of limited cases.

Archimedes' mathematical approach to physics, especially, became a model for subsequent scientists such as Guidobaldo del Monte, Bernardino Baldi, Simon Stevin, and Galileo Galilei.

[13][14] The concept of centre of gravity reached a high level of sophistication in the first half of the seventeenth century, particularly in the works of Evangelista Torricelli and Christiaan Huygens, and played a pivotal role in the development of rational mechanics.

[9] There is also no definition of the centre of gravity anywhere in Archimedes' extant works, which some scholars argue makes it difficult to follow (or justify) the logical structure of some of his arguments in On the Equilibrium of Planes.