This chapter begins to investigate the puzzle of why Western European states have historically permitted, and to a considerable extent continue to abet, relatively high levels of immigration. Although these questions will be considered more fully in later chapters, we will address them here in the context of the origins and characteristics of post-WWII migration to Western Europe. Before we do so, however, two facts should be underscored. First, postwar immigration did not unfold in a single seamless pattern but, rather, in overlapping waves (1945–79, 1973–2007, 1989–2007). As we will see in this chapter, each of the three major waves – labor immigration, family reunification, and irregular/forced immigration – was precipitated by a unique set of short-term stimuli and governed by its own peculiar logics. The second fact that should be highlighted at the outset of this chapter is that the aforementioned waves of immigration are inextricably linked. Even a cursory examination of the historical and contemporary patterns of migration to the immigrant-receiving countries leads to the inescapable conclusion that the efforts (successful or not) of states to restrict the number of persons entering their territory along any one dimension of migration increase the number of immigrants seeking and gaining entry along other dimensions.