  In this paper, we discuss the theory of the Siegel modular variety in the
aspects of arithmetic and geometry. This article covers the theory of Siegel
modular forms, the Hecke theory, a lifting of elliptic cusp forms, geometric
properties of the Siegel modular variety, (hypothetical) motives attached to
Siegel modular forms and a cohomology of the Siegel modular variety.
