A new book by Professor Paul Collier is published by Penguin (in the UK) and OUP (in the USA). The book, entitled "Exodus: How migration is changing our world", clearly lays out the effects of engouraging or restricting migration. Drawing on original research and case studies, Collier explores the challenges and debates around migration from three perspectives: the migrants, the people left behind and the host societies. Here, Prof Collier gives some reflections on why he wrote the book and what kinds of debate he hopes it will open up.