  A self-contained pedagogical introduction to the functional Schr\"{o}dinger
picture method of many-body theory is given at a level suitable for graduate
students and also for many-body physicists who have not been exposed to the
functional Schr\"{o}dinger picture method previously. Mathematical tools
necessary for the functional Schr\"{o}dinger picture calculation are
introduced. The method is first applied to various condensed matter problems
including the electron gas, the Hubbard model, the BCS superconductivity, and
dilute bose gas within the variational approximation. It is shown that the
variational approximation with the Gaussian trial functional invariably leads
into the Hartree-Fock results for both zero and finite temperature cases.
  In order to go beyond the Gaussian results, concepts of variational and
optimized perturbation theories are introduced using simple quantum mechanical
languages. Then, the variational perturbation theory in the functional
Schr\"{o}dinger picture is applied to the $\lambda \phi^4$ model to yield the
effective potential to the third order. The optimized perturbation theory is
also applied to the $\lambda \phi^4$ model to yield a general expression up to
the second order.
