There are REUs in many scientific fields such as mathematics, physics, chemistry, geology, biology, psychology, and computer science.
REU sites typically consist of ten undergraduates working in the research program of the host institution either in the US or abroad, for example, CERN.
Sometimes these undergraduates work with a larger research team that includes graduate students.
These REU experiences take place at the student's current university, and can last anywhere from a few weeks to an entire year.
One long-running REU-site in the 1990s was Washington University's Computer Science SURA Program.