This thesis analyses how 'public opinion' was conceptualised by Members of the Swedish Parliament (MPs) between 1848 and 1919. The source material consists of the printed minutes from parliamentary debates where issues such as religious freedom, constitutional reform and reform of the Press were discussed. What happened to the ideal of an enlightened public opinion when the development of a large-scale industrial economy changed the nature of the Press? Two main aspects of public opinion are analysed. Firstly, the question of what MPs considered the most reliable source of public opinion is examined. The legitimacy of manifestations claiming to represent public opinion, such as written petitions, the Press, Parliament itself, quantitative estimations and also the silent opinion was discussed. In the 1910s the voices of women were also included by some MPs when assessing public opinion. The second main aspect is how MPs envisioned the relationship between the reliability of public opinion and the conditions for public discourse. Here an important distinction was made between public opinion formed in a free and unhindered debate and that brought about by persuasion. The study shows that public opinion was a contested concept in the Swedish Parliament. In the 1850s, Conservatives gave the religiously conservative nature of public opinion as a reason to postpone the reform of religious laws. In debating constitutional reform, on the other hand, it was the Liberals who argued that decisions should follow public opinion. In the 1910s, the Left was divided over the relationship between public opinion and the State, with some arguing that the State should intervene in the public debate to offset the negative influence of market mechanisms. Others felt that public opinion rather than legislation should set the limits of the public discourse, especially in the case of religion, but also concerning the Press.