This paper examines the specific case of migration flows from Poland to the UK in the wake of the 2004 EU enlargement. Beginning with an analysis of the prevailing theories of migration, which attempt to both describe and predict the phenomenon, the paper moves on to an empirical analysis of migration flows in both Poland and the UK in the years immediately following enlargement. The effects of these flows on the internal market of the UK are examined through a comparison with other exogenous migration shocks that have impacted other countries in recent history. Finally, an attempt is made to evaluate the prevailing theories analyzed earlier in light of the empirical evidence.