In linguistics, an interrogative word or question word is a function word used to ask a question, such as where or what. They are sometimes called wh-words, because in English most of them start with wh-. 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 their 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).

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, and czy in Polish. The English word whether has a similar function, but only in indirect questions.

Interrogative words in English include:

A frequent class of interrogative words in several other languages is the interrogative pro-verb: