  We discuss an attempt to understand confinement and dynamical symmetry
breaking in a 4-dimensional non-supersymmetric gauge theory using S-duality in
type IIB string theory. The electric theory is a USp(2n) gauge theory and its
magnetic dual is argued to be an SO(2n) or SO(2n-1) gauge theory. These
theories are obtained as the low-energy effective theory of O3-D3bar systems in
type IIB string theory, which are related by S-duality. Confinement and
dynamical symmetry breaking in the USp(2n) gauge theory are caused by the
condensation of scalar fields in the magnetic dual description, which is
consistent with the scenario of the dual Meissner mechanism for the
confinement.
