Nasalization is the production of speech sounds by emitting air through the nasal cavity. Air traveling up from the lungs is modified at different points by various structures to produce the different sounds used in speech. As the air flows up, it can be diverted into either the oral or the nasal cavity. Three consonant sounds in English are characterized by nasalized air flow. English vowels are only nasalized when they occur near a nasalized consonant in a word.

The velum, or soft palate, is a muscle located in the back of the roof of a person's mouth. It facilitates or prevents the nasalization of speech sounds by channeling air into either the oral or the nasal cavity. When closed, the velum seals off the nasal cavity for the production of oral speech sounds, open it does the opposite to facilitate nasal sounds. Breathing requires an open velar position so the velum remains open at rest. Closing it to produce oral speech sounds requires active muscle control.

English features three nasalized consonants: n, m, and ng. These three sounds are produced using oral articulators, such as the lips for m, even though the air flows predominantly through the nasal cavity. Although there are no English vowels with nasalization as a distinctive feature, vowels in proximity to nasal consonants will often become nasalized during normal speech production due to a process called assimilation. Listeners perceive failure to nasalize ordinarily assimilated vowels as mispronunciations. Other languages, such as French, feature purely nasalized vowels.

Many children exhibit nasalization errors as they learn to speak. For example, a child may nasalize the beginning of a word that ends in a nasalized sound, or vice versa, such as substituting "man" for "tan" or "man" for "mat." Assimilation errors such as these are easily identified based on communication contexts and typically resolve themselves in early childhood without intervention. Errors of this type that persist past the age of five years should be evaluated by a speech-language pathologist.

Disorders such as cleft palate often result in the overproduction of nasalized speech sounds. In many cases palatal disorders affect the velar musculature and make it difficult to raise the velum and close off the nasal cavity. This results in normally oral sounds being produced nasally which distorts the sound and frequently makes the person's speech difficult to understand. Surgery is often required to correct this type of nasalization error.