Book excerpt: What do we do when we are about to utter speech? On which linguistic units do we rely? How do these units evolve from childhood to adulthood, or across time for a given language? How do we assemble these units under the influences of syntactic, phonological and prosodic rules? Do we plan the whole sequence at once? Do we plan the movements of the tongue, jaw, and lips underlying speech in the same way that we plan other movements? What tools have scientists developed to investigate these questions using observation of articulatory and acoustic signals? This book addresses these issues in eight chapters. Written by specialists in the field, these chapters provide the readers with a large overview of the literature, and illustrate the research challenges using selected examples of experimental studies.