Since about 1900, public opinion has generally been defined as a psychosocial phenomenon. We want to return to a more political definition because it provides a more synthetical view of it.As such, it requires to be analyzed essentially in dialectical contrast with decision making.Different aspects may be distinguished in it, such as «opinion families» and «opinion movements». Their processes are not quite identical, altough they are simultaneous.Furthermore, public opinion exists everywhere. But it follows different ways whether it appears in hierarchical or in egalitarian institutions.