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We review in a common framework several algorithms that have been proposed recently, in order to improve the voice quality of a text-to-speech synthesis based on acoustical units concatenation (Charpentier and Moulines, 1988; Moulines and Charpentier, 1988; Hamon et al., 1989). These algorithms rely on a pitch-synchronous overlap-add (PSOLA) approach for modifying the speech prosody and concatenating speech waveforms. The modifications of the speech signal are performed either in the frequency domain (FD-PSOLA), using the Fast Fourier Transform, or directly in the time domain (TD-PSOLA), depending on the length of the window used in the synthesis process. The frequency domain approach is capable of a great flexibility in modifying the spectral characteristics of the speech signal, while the time domain approach provides very efficient solutions for the real time implementation of synthesis …