This paper presents real-time musical applications using the IRCAM signal processing workstation which make use of FFT/IFFT-based resynthesis for timbral transformation in a musical context. An intuitive and straightforward user interface, intended for use by musicians, has been developed by the authors in the Max programming environment. Techniques for high quality time-stretching, filtering, cross-synthesis, dynamic range processing, and spectrum shaping are presented along with dynamic control structures that allow for both fine timbral modification and control of complex sound transformations using few parameters.