The rise of anti-immigration parties across Western Europe has put enormous pressure on mainstream parties to adapt their competitive strategies. This thesis tests the hypothesis that mainstream parties have reacted to the rise of an anti-immigration party by taking up more restrictive immigration positions. Previous research on the impact of the rise of an anti-immigration party on immigration positioning is sparse, and centres mostly on the supply side of the political market using one-dimensional left-right explanations, which generally do not provide a satisfactory framework to explain party movement. This thesis is based on a spatial two-dimensional model of political competition and combines analyses of the demand and the supply side of the political market. Focusing on the cases of the Netherlands, Flanders and the UK in the time-period 1987-2010 it shows that parties ultimately respond to electoral pressure when choosing their immigration strategy. The thesis has three main findings. Firstly, voters perceive their parties to have too liberal immigration preferences, causing a persistent anti-immigrant gap to exist. In combination with high divisiveness of the immigration issue for mainstream party constituencies the antiimmigrant gap provides opportunities for an anti-immigration party to emerge strongly in multi-party systems. Secondly, the electoral market on immigration is characterised by conflicting incentives, which makes successfully reacting to an anti-immigration party very difficult, especially for traditional mainstream parties. Thirdly, patterns of political competition on the immigration dimension reflect these conflicting incentives. On the one hand, the high divisiveness of the issue at party level urges mainstream parties of left, centre and right equally to depoliticise. Accordingly, parties tend to compete with relatively liberal immigration positions in the absence of a credible threat by an anti-immigration party. On the other hand, a correlation between the left-right and the immigration dimension on the demand side of the political market at party system level creates a pull towards restrictionism, which is stronger for parties on the right. Once an anti-immigration party presents a credible threat parties indeed react by moving their immigration position closer to that of the anti-immigration party following a left-right logic, sometimes dramatically so. The party system strongly impacts the dynamics of indirect impact: in the UK mainstream parties never lose the initiative on immigration to an anti-immigration party.