This chapter will describe some basic concepts necessary to understand the spectral envelopes project. After a short introduction to some concepts in digital signal processing which permeate the whole project in section 2.1, the basic ideas of additive sound analysis and synthesis are explained in section 2.2, followed by a closer examination of the concept of spectral envelopes. Finally, to embed the project in the ongoing work at IRCAM, the programs used here, and the programs and systems where spectral envelope manipulation will be used will be visited briefly in section 2.5.