Illegal alien, also called an undocumented alien, is a noncitizen living in a country without the proper immigration documents. Some illegal aliens have fled poverty, war, starvation, and persecution in their native lands. Others are seeking better job opportunities in countries with stronger economies. Illegal immigration has been a growing problem for governments since the early 1900's. The nations most affected by illegal aliens are those with a high standard of living: the United States, Canada, and the nations of Western Europe.