Yes and no, or word pairs with a similar usage, are expressions of the affirmative and the negative, respectively, in several languages including English. Some languages make a distinction between answers to affirmative versus negative questions, thus they may have triplets or quadruplets of words instead. English originally used a four-form system up to and including Early Middle English but Modern English has reduced this to a two-form system consisting of just 'yes' and 'no'. It exists in many facets of communication, such as: eye blink communication, head movements, Morse Code, and sign language.

Some languages do not answer yesses with single words meaning 'yes' or 'no'. Welsh, Finnish and Chinese are among several languages that typically employ echo answers (repeating the verb with either an affirmative or negative form) rather than using words for 'yes' and 'no', though both languages do also have words broadly similar to 'yes' and 'no'. Other languages have systems named two-form, three-form, and four-form systems, depending on how many words for yes and no they employ. Some languages, such as Latin, do not have yes-no word systems.

The words yes and no are not easily classified into any of the eight conventional parts of speech. Although sometimes classified as interjections, they do not qualify as such, and they are not adverbs. They are sometimes classified as a part of speech in their own right, sentence words, word sentences, or pro-sentences, although that category contains more than yes and no and not all linguists include them in their lists of sentence words. Sentences consisting solely of one of these two words are classified as minor sentences.

The differences among languages, the fact that in different languages the various words for yes and no have different parts of speech and different usages, and that some languages lack a 'yes-no' word system, makes idiomatic translation very difficult.