Over the last three decades many Western European social democratic parties have been challenged by populist radical right parties. The growth and success of parties on the right flank of the party system represents a triple challenge to the social democrats: they increase the salience of issues traditionally ‘owned’ by the right; they appeal to working-class voters who traditionally support the centre left; and they may facilitate the formation of centre-right governments. This article explores social democratic parties' strategic options in the face of this challenge, and tests the widespread assumption that the centre-left parties respond by taking a tougher stance on issues related to immigration and integration. Comparative analysis of developments in Austria, Denmark, the Netherlands and Norway reveals significant variation in the substance, scope and pace of the strategic responses of their social democratic parties. And it suggests that those responses are influenced not only by the far right but also by the reactions of mainstream centre-right parties and by parties on their left (and liberal) flank. Internal disunity, potential or actual, is also an important factor.