  These lecture notes are based on three lectures, each ninety minutes long,
given by the author at the "International School on Strings and Fundamental
Physics" which took place in Garching/Munich from July 25 to August 6, 2010.
These lectures, aimed at graduate students, require only a basic knowledge of
string theory and give a simple introduction to double field theory. These
notes were prepared by Marco Baumgartl and Nicolas Moeller.
