This book grew out of three series of lectures given at the summer
school on "Modular Forms and their Applications" at the Sophus Lie
Conference Center in Nordfjordeid in June 2004. The first series treats
the classical one-variable theory of elliptic modular forms. The second
series presents the theory of Hilbert modular forms in two variables and
Hilbert modular surfaces. The third series gives an introduction to
Siegel modular forms and discusses a conjecture by Harder. It also
contains Harder's original manuscript with the conjecture.
Each part treats a number of beautiful applications.