Comparative law is the study of differences and similarities between the law and legal systems of different countries. More specifically, it involves the study of the different legal systems (or "families") in existence around the world, including common law, civil law, socialist law, Canon law, Jewish Law, Islamic law, Hindu law, and Chinese law. It includes the description and analysis of foreign legal systems, even where no explicit comparison is undertaken. The importance of comparative law has increased enormously in the present age of internationalism and economic globalization.

The origins of modern comparative law can be traced back to Gottfried Wilhelm Leibniz in 1667 in his Latin-language book Nova Methodus Discendae Docendaeque Iurisprudentiae (New Methods of Studying and Teaching Jurisprudence). Chapter 7 (Presentation of Law as the Project for all Nations, Lands and Times) introduces the idea of classifying Legal Systems into several families. A few years later, Leibniz introduced an idea of Language families.

Although every legal system is unique, comparative law through studies of their similarities and differences allows for classification of legal systems, wherein law families is the basic level of the classification. The main differences between law families are found in the source(s) of law, the role of court precedents, the origin and development of the legal system. Montesquieu is generally regarded as an early founding figure of comparative law. His comparative approach is obvious in the following excerpt from Chapter III of Book I of his masterpiece, De l'esprit des lois (1748; first translated by Thomas Nugent, 1750):

[T]he political and civil laws of each nation ... should be adapted in such a manner to the people for whom they are framed that it should be a great chance if those of one nation suit another.

They should be in relation to the nature and principle of each government: whether they form it, as may be said of politic laws; or whether they support it, as in the case of civil institutions.

They should be in relation to the climate of each country, to the quality of its soil, to its situation and extent, to the principal occupation of the natives, whether husbandmen, huntsmen, or shepherds: they should have relation to the degree of liberty which the constitution will bear; to the religion of the inhabitants, to their inclinations, riches, numbers, commerce, manners, and customs.

Also, in Chapter XI (entitled 'How to compare two different Systems of Laws') of Book XXIX, discussing the French and English systems for punishment of false witnesses, he advises that "to determine which of those systems is most agreeable to reason, we must take them each as a whole and compare them in their entirety." Yet another place where Montesquieu's comparative approach is evident is the following, from Chapter XIII of Book XXIX:

As the civil laws depend on the political institutions, because they are made for the same society, whenever there is a design of adopting the civil law of another nation, it would be proper to examine beforehand whether they have both the same institutions and the same political law.