Capitalization (American English) or capitalisation (British English) is writing a word with its first letter as a capital letter (uppercase letter) and the remaining letters in lower case, in writing systems with a case distinction. The term also may refer to the choice of the casing applied to text.

Conventional writing systems (orthographies) for different languages have different conventions for capitalization, for example, the capitalization of titles. Conventions also vary, to a lesser extent, between different style guides. In addition to the Latin script, capitalization also affects the Armenian, Cyrillic, Georgian and Greek alphabets.

The full rules of capitalization in English are complicated. The rules have also changed over time, generally to capitalize fewer words. The conventions used in an 18th-century document will be unfamiliar to a modern reader; for instance, many common nouns were capitalized.

The systematic use of capitalized and uncapitalized words in running text is called "mixed case".