Unlike most western languages, Japanese has an extensive grammatical system to express politeness and formality.

Broadly speaking, there are three main politeness levels in spoken Japanese: the plain form (kudaketa 砕けた or futsuu 普通), the simple polite form (teineigo 丁寧語) and the advanced polite form (keigo 敬語).

Since most relationships are not equal in Japanese society, one person typically has a higher position. This position is determined by a variety of factors including job, age, experience, or even psychological state (e.g., a person asking a favour tends to do so politely). The person in the lower position is expected to use a polite form of speech, whereas the other might use a more plain form. Strangers will also speak to each other politely. Japanese children rarely use polite speech until they are teens, at which point they are expected to begin speaking in a more adult manner. See uchi-soto

The plain form in Japanese is recognized by the shorter, dictionary form of verbs, and the da form of the copula. At the teinei level, verbs end with the helping verb -masu, and the copula desu is used. The advanced polite form, keigo, actually consists of two kinds of politeness: honorific language (sonkeigo) and humble (kenjōgo) language. Whereas teineigo is an inflectional system, keigo often employs many special (often irregular) honorific and humble verb forms: iku "to go" becomes ikimasu in polite form, but is replaced by mairimasu in humble form and irrashaimasu in honorific form.

The difference between honorific and humble speech is particularly pronounced in the Japanese language. Humble language is used to talk about oneself or one's own group (company, family) whilst honorific language is mostly used when describing the interlocutor and his group. For example, the -san suffix ("Mr", "Mrs" or "Ms") is an example of honorific language. It is not used to talk about oneself or when talking about someone from one's company to an external person, since the company is the speaker's "group".

Most nouns in the Japanese language may be made polite by the addition of o- or go-; as a prefix. o- is generally used for words of native Japanese origin, whereas go- is affixed to words of Chinese derivation. In some cases, the prefix has become a fixed part of the word, and is included even in regular speech, such as gohan 'cooked rice; meal.' Such a construction often indicates deference to either the item's owner or to the object itself. For example, the word tomodachi 'friend,' would become o-tomodachi when referring to the friend of someone of higher status (though mothers often use this form to refer to their children's friends). On the other hand, a female speaker may sometimes refer to mizu 'water' as o-mizu merely to show politeness; this contrasts with the more abrupt speech of men (though men may also use very polite forms when speaking to superiors).

Most Japanese people employ politeness to indicate a lack of familiarity. That is, they use polite forms for new acquaintances, but if a relationship becomes more intimate, they no longer use them. This occurs regardless of age, social class, or gender.

Many researchers report that since the 1990s, the use of polite forms has become rarer. Needless to say, many older people disapprove of this trend. Young people usually receive extensive training in the "proper" use of polite language when they start to work for a company.