Migration is a minefield for demographers; varying definitions and meaurements can be difficult to sort out. Migration is the geographic movement of people across a boundaries to establish a new permanent or semipermanent residence. Along with fertility and mortality, migration is a component of population change. How is an "immigrant" defined? Why do people migrate? How is the world becoming a "melting pot?" In this video, PRB senior demographer Carl Haub answers these questions and clarifies how migration affects population.