Immigration to OECD countries has been rising since the 1960s, albeit with numerous ups and downs. This trend continued into the first decade of the new century, which saw the number of immigrants living in OECD countries rise from around 75 million at the start of the decade to more than 100 million at its close. Mexico, China, the United Kingdom and India are the biggest countries of origin. Behind this long-term trend lies considerable diversity, both in flows to individual countries and between different immigration categories, such as labour migration, family migration and humanitarian migration.