"Brain drain" is a perception that is hard to measure. In 2000, the US Congress announced it was raising the annual cap on the number of temporary work visas granted to highly skilled professionals under its H1B visa program, from 115,000 to 195,000 per year, effective until 2003. That suggests a ballpark figure for the influx of talent into the United States at that time. In the same year the British government cooperating with the Wolfson Foundation, a research charity, launched a £20 million, five-year research award scheme that aimed at drawing the return of the UK’s leading expatriate scientists and sparking the migration of top young researchers to the United Kingdom.

Historically, the greatest brain drains have been from rural to urban areas. In the 19th century and 20th century there were great migrations to North America from Europe, and in modern times, from developing nations to developed nations, especially after colonialism. Sometimes such drains occur between developed nations, e.g. from Canada to the United States especially in the finance, software, aerospace, healthcare and entertainment industries due to higher wages and lower taxes. Only in recent years this trend seems to be slowing and even reversing with many Canadians returning home to work.

Iraq is said by some to be presently undergoing a brain drain due to its political instability. A report by The Washington Prism in Jan 2006 claims that the International Monetary Fund considers Iran ranked highest in Brain Drain among developing countries.

Some scientists say that, with the low amount of engineers percentage-wise produced in the US today, the US is experiencing a type of culture-braindrain.

The former Soviet Union and today's Russia continues to experience a braindrain in science, business, and culture as to many of its Jewish citizens leaving for the United States of America and Israel because of historical Russian anti-Semitism.

New Zealand has also experienced somewhat of a braindrain for a variety of reasons, and many University graduates in that nation are lured to neighboring Australia where wages are generally higher and economic opportunity is more diverse. The Trans-Tasman Travel Arrangement makes such exchanges very easy, although it is heavily one-sided as mostly Kiwis (New Zealanders) move to Australia while few well-qualified Australians move to New Zealand.

This phenomenon is perhaps most problematic for developing nations, where it is widespread. In these countries, higher education and professional certification are often viewed as the surest path to emigration from a troubled economy or political situation. This has led to situations such as that in many Latin American nations, where enrollment at medical schools is very high, but the nation has a chronic shortage of doctors.

An opposite situation, in which many trained and talented individuals seek entrance into a country, is called a brain gain; this may create a brain drain in the nations that the individuals are leaving. A Canadian symposium in 2000 gave circulation to the new term, at a moment when many highly-skilled Canadians were moving to the United States but, simultaneously, many more qualified immigrants were coming to Canada. This is sometimes referred to as a 'brain exchange'.