Theoretical approaches to public opinion are hard to find in the sociological literature, with the exception of the seminal work of Jürgen Habermas. One important alternative, although almost unknown in the English-speaking world, is offered in a few contributions by the systems theoretician Niklas Luhmann. Both critical theory and systems theory start from a historical analysis of the conditions that led to the rise of a public sphere and understand its function as the limitation and control of the arbitrariness of (...) power. Critical theory considers the public sphere as a social space where citizens can (or should) participate and discuss freely and without constraints. Thus, it legitimizes political power. Systems theory presents a completely different concept of the public sphere and conceives of it in terms of second-order observation. Through public opinion the modern political system observes itself and stimulates as well as limits its decision-making processes. This paper argues that both approaches share the idea that the political system, like every other social subsystem, must generate a system-specific uncertainty (i.e. specific conditions that it cannot control) in order to limit its own arbitrariness and to be able to develop its decision-making potential. Both approaches locate this uncertainty in the sphere of public opinion. But they radically differ in the way they conceptualize public opinion’s effects on modern politics. Such differences between critical theory and systems theory are illustrated by an analysis of recent political events. (shrink)