In this PhD thesis we are investigating topics on international migration and we focus in the case of the UK as a host country. We analyse the implications of restricted and unrestricted migration on the labour market outcomes and the welfare use of the migrants. In the first chapter we estimate the joint decision over the labour market behavior of an individual and her subsequent welfare use, and attempt to explain how this decision is differentiated between natives and immigrants. We incorporate differences in the purchasing power parities of the home countries and the host country to explain how these differences create different incentives between natives and immigrants. In the second chapter we investigate the effects of the economic crisis on the labour market performance of natives and immigrants in the UK. We assess the unemployment durations of EU and non-EU immigrants, and UK natives for the years before and during the economic crisis of 2008. We find that the unemployment duration of the EU immigrants converged to that of the UK natives, while the non-EU immigrants were the ones affected the most. We also find that the high degree of clustering into specific socioeconomic statuses, drove at a significant degree the unemployment duration out-comes for the EU immigrants. In the third chapter we investigate the job separation rates of immigrants and natives for the periods before and during the economic crisis of 2008. We find that the non-EU immigrants had higher separation rates than the natives and that this gap widened even further for the years during the crisis. The A8 immigrants had higher separation rates than the natives mainly due to early attrition from the survey. Exits towards unemployment or underemployment happened at a lower rate for this immigrant group compared to the natives. The two results combined indicate the importance of out-migration when the labour market outcomes of EU migrants are compared to those of the UK natives.