An interrogative word or question word is a function word used to ask a question, such as what, which, when, where, who, whom, whose, why, whether and how. They are sometimes called wh-words, because in English most of them start with wh- (compare Five Ws). They may be used in both direct questions (Where is he going?) and in indirect questions (I wonder where he is going). In English and various other languages the same forms are also used as relative pronouns in certain relative clauses (The country where he was born) and certain adverb clauses (I go where he goes). It can also be used as a modal, since question words are more likely to appear in modal sentences, like (Why was he walking?)

A particular type of interrogative word is the interrogative particle, which serves to convert a statement into a yes–no question, without having any other meaning. Examples include est-ce que in French, ли li in Russian, czy in Polish, чи chy in Ukrainian, ĉu in Esperanto, āyā آیا in Persian, কি ki in Bengali, 嗎/吗 ma in Mandarin Chinese, mı/mi in Turkish, pa in Ladin, か ka in Japanese, 까 kka in Korean, ko/kö in Finnish and (да) ли (da) li in Serbo-Croatian. "Is it true that..." and "... right?" would be a similar construct in English. Such particles contrast with other interrogative words, which form what are called wh-questions rather than yes–no questions.

For more information about the grammatical rules for using formed questions in various languages, see Interrogative.