The Spirit of the Laws (French: De l'esprit des lois, originally spelled De l'esprit des loix; also sometimes called The Spirit of Laws) is a treatise on political theory first published anonymously by Charles de Secondat, Baron de Montesquieu, in 1748 with the help of Claudine Guérin de Tencin.

Montesquieu has spent around twenty-one years researching and writing De l'esprit des lois, covering such topics as law, social life, and the study of anthropology and providing more than three thousand commendations.

In this political treatise Montesquieu pleads in favor of a constitutional system of government and the separation of powers, the ending of slavery, the preservation of civil liberties and the law, and the idea that political institutions ought to reflect the social and geographical aspects of each community.

In his classification of political systems, Montesquieu defines three main kinds: republican, monarchical, and despotic.

As he defines them, Republican political systems vary depending on how broadly they extend citizenship rights—those that extend citizenship relatively broadly are termed democratic republics, while those that restrict citizenship more narrowly are termed aristocratic republics.

The distinction between monarchy and despotism hinges on whether or not a fixed set of laws exists that can restrain the authority of the ruler: if so, the regime counts as a monarchy; if not, it counts as despotism.

A second major theme in De l'esprit des lois concerns political liberty and the best means of preserving it.

"Political liberty" is Montesquieu's concept of what we might call today personal security, especially insofar as this is provided for through a system of dependable and moderate laws.

He distinguishes this view of liberty from two other, misleading views of political liberty.

The first is the view that liberty consists in collective self-government—i.e. that liberty and democracy are the same.

The second is the view that liberty consists in being able to do whatever one wants without constraint.

Not only are these latter two not genuine political liberty, he thinks, they can both be hostile to it.

Political liberty is not possible in a despotic political system, but it is possible, though not guaranteed, in republics and monarchies.

Generally speaking, establishing political liberty on a sound footing requires two things: The separation of the powers of government, and the appropriate framing of civil and criminal laws so as to ensure personal security.

The third major contribution of De l'esprit des lois is to the field of political sociology, which Montesquieu is often credited with more or less inventing.

The bulk of the treatise, in fact, concerns how geography and climate interact with particular cultures to produce the spirit of a people.

This spirit, in turn, inclines that people toward certain sorts of political and social institutions, and away from others.

Originally published anonymously partly because Montesquieu's works are subject to censorship, its influence outside France will be aided by its rapid translation into other languages.