The aim of this thesis is to investigate the understudied nexus between immigration and labour market institutions. Bridging migration theory and comparative political economy, it is argued that variation in labour market institutions shapes variation in levels of labour immigration and the type of employment undertaken by migrant workers across advanced democracies. This theory is supported by empirical evidence from Western Europe and in particular Italy and Ireland. Furthermore, it is explored whether, given the relationship between labour market institutions and labour immigration, policymakers are stimulated by concerns regarding immigration - the ‘Migrant Worker Factor’ - to reform labour market institutions. More specifically, it is investigated whether the Migrant Worker Factor stimulated employment standards compliance (ESC) system reforms and the development of active labour market policy (ALMP) in Ireland, Italy and shadow cases Britain and France during the period 1997 - mid 2008. It is also explored whether the Migrant Worker Factor was part of the rationale behind vocational education and training (VET) system reforms of the same period in Ireland and Britain. These case studies found empirical evidence that concerns regarding labour immigration can be a causal factor in the reform of labour market institutions. Certain ESC system reforms were partly motivated by the aim of safeguarding domestic worker jobs and employment standards, as well as controlling migrant inflows. Moreover, in some cases, ALMP development and VET system reforms in part aimed to provide domestic alternatives to migrant workers. The impact of the Migrant Worker Factor depends on the level of concern regarding labour immigration in a given state. Furthermore, the influence of this variable on labour market institutional reform is mediated by learning and coordination mechanisms and assumptions regarding the substitutability of migrant workers. The Migrant Worker Factor is also more likely to stimulate labour market institutional reform when there are converging political or structural pressures for reform.