The quality of a speech synthesizer is judged by its similarity to the human voice, and by its ability to be understood. Alternatively, a synthesizer can incorporate a model of the vocal tract and other human voice characteristics to create a completely 'synthetic' voice output. For specific usage domains, the storage of entire words or sentences allows for high-quality output. Systems differ in the size of the stored speech units a system that stores phones or diphones provides the largest output range, but may lack clarity.

Synthesized speech can be created by concatenating pieces of recorded speech that are stored in a database. A text-to-speech (TTS) system converts normal language text into speech other systems render symbolic linguistic representations like phonetic transcriptions into speech.

A computer system used for this purpose is called a speech synthesizer, and can be implemented in software or hardware. Speech synthesis is the artificial production of human speech.