Undocumented international migration is an increasingly important political, social and economic issue. The articles collected in this volume provide a framework for the study of some key decisions that potential migrants are confronted with when considering a move abroad. This includes the timing of departure, the method of financing the move, the choice between documented and undocumented modes of entry, the optimal duration of the stay abroad, how much to save, etc. The various chapters illustrate how decisions of migrants are shaped not only by immigration policies and enforcement measures of the host country, but also by their own personal characteristics and the economic environment they face at home and abroad. At the macroeconomic level, the focus is on the analysis of the effectiveness of immigration policies in controlling the inflow and the stock of undocumented aliens. The question of international cooperation between the host and transit countries is also examined.