Movement by persons across internal boundaries or international frontiers may take place without legal process. The reasons may include the need for secrecy, which may apply to criminals; the need to flee another country with no time to meet legal requirements; and the desire to enter a country although permission will not be granted, either to leave or to enter. The intentions of movement may allow return, or they may be one way in the case of illegal refugees or migrants. Legal obstruction to movement across frontiers may cause serious crimes, from forgery, to stolen vehicles or aerial hijackings, to murder. Those in movement may also become helpless victims of mistreatment or murder by border guards, or victims of bandits and other criminals.
Large scale or recurring illegal movements have been from eastern Europe, east Berlin, Kampuchea, Vietnam, Cuba, mainland China, Mexico, India and Pakistan, among others. Countries with an illegal immigrant problem include the USA, the UK, Germany, Netherlands, France and Belgium. In the UK, for example, some 1,600 illegal immigrants were deported in 1983. At the USA-Mexico border, thousands of Mexicans commute daily to jobs in the USA, although such "commuting" is for the most part illegal.