Most authors agree that modal particles – a class of function words widely considered characteristic of Modern German – cannot receive prosodic stress, though the reasons for this restriction have not yet been satisfactorily explained. This paper argues that unstressability follows from the general contribution of modal particles to compositional utterance meaning, which requires them to take scope over focus-background structures. Form and function of modal particle meanings are modelled and illustrated for five representative examples – the particles wohl, ja, eigentlich, eben and halt. It is argued that these as well as other particles, whenever they occur under prosodic stress, cannot preserve the meaning nor the syntactic behaviour of modal particles. All instances of stressed particles in German must therefore be categorized in other functional classes.