Understanding how political parties and social movements interact and what are the results of these interactions is important for both scholars of Political Science and Sociology. This is particularly true for far-right actors since they are characterised by diverse ideological and organisational features. Understanding how parties and movements interact can help shed light on how these features develop and, ultimately, explain their success. In the dissertation a novel conceptualisation of party-movement interactions is presented, as well as a theory that aims to explain when parties and movements are more likely to develop stronger interactions on three different dimensions, frames, actions and organisations. This theory is tested by looking at two sets of far-right parties and movements, Lega Nord and CasaPound Italia in Italy and the United Kingdom Independence Party and the English Defence League in the United Kingdom. The analysis is carried out through a Political Claim Analysis and a document analysis of parties and movements documents for the period 2009-2019. The analysis finds that parties and movements have closer interactions on the frame dimension when issues they own gain prominence in the public debate and when political parties are weak electorally. In the actions dimension, interactions tend to be closer when parties are weak electorally and movement organisations moderate their repertoire of actions. Finally, in the organisational dimension, relations are closer when parties are weak electorally and in proximity of electoral campaigns. This research makes two contributions to the study of far-right parties and movements. The first is theoretical, for the paper advances a new theory of party-movement interactions that could be tested in different scenarios. The second is empirical, for the paper provides indications on when parties and movements are more likely to have closer interactions and how through these interactions they change and develop their features.