Societies today are increasingly characterized by their ethnic, racial, and religious diversity. One key question raised by the global migration of people is how they do or do not come to be incorporated into their new social environments. For over a century, assimilation has been the concept used in explaining the processes of immigrant incorporation into a new society. It has also been applied to indigenous peoples, to refugees, and to involuntary migrants caught up in the slave trade. Assimilation has confronted many scholarly challenges which were often intermeshed with particular political agendas.