Book excerpt: The well known Selberg trace formula reduces the problem of calculating the dimension of cusp forms of Siegel upper-half plane, when the fundamental domain is not compact but has finite volume, to the evaluation of certain integrals combining with special values of certain zeta functions. In this paper, we shall obtain explicit dimension formulae for cusp forms of degree three with respect to the full modular group Sp(3, [bold]Z) and its principal congruence subgroups by a long computation.