In this thesis, I scrutinise and interpret Jürgen Habermas’s claim that justification of moral norms necessitates cognitivism. I do this by analysing the general idea behind his discourse theory of morality and then his metaethics. From there, I examine the non-cognitivist theory called prescriptivism as set out by Richard Hare to see if his account of moral reasoning is able to counter Habermas’s claims and thereafter, I examine some criticism against his concept of communicative action. I also engage with the discussion on how to define cognitivism: that is, whether the line should be drawn between moral realism on the cognitivist side, and constructivism on the other, or if cognitivism can include constructivist theories too. I propose that it should, provided that it allows moral statements to be truth-apt and express a mental state like that of belief. Following this definition, I argue that Habermas can be labelled a cognitivist and finally, I conclude that Habermas's argument does not hold under scrutiny.