This document discusses key topics related to migration including definitions, types of migration, theories of migration, trends in international migration, and challenges. It defines various types of migration such as international, internal, circular, and net migration. Theories of migration discussed include early macro theories, neoclassical economic theories, and socio-political network theories. International migration is at an all-time high and trends include half of migrants moving between developing countries and some countries shifting from emigration to immigration. Prospects are that international migration rates will remain high due to economic and population disparities between countries. Challenges include integration of immigrants in receiving countries and "brain drain" issues in sending countries.