This book looks at the background to the policy of free movement in Europe and discusses the consequences. European integration changed migration conditions considerably: Under the concept of "freedom of movement", border crossings between EU member states as well as work and settlement by nationals of other member states were largely facilitated; internal borders thus lost their significance. At the same time, the question of how to deal with a common external border and the migration of "third-country nationals" gained in importance. The essential explains why migration from outside Europe was increasingly understood as a problem of security policy and why this still determines the measures for designing a common external border today.