Spontaneous symmetry breaking at finite temperature is studied.
We show that for the class of theories discussed, symmetry is restored above a critical temperature. We determine this by a functional-diagrammatic evaluation of the effective potential and the effective mass. A formula is obtained in terms of the renormalized parameters of the theory. By examining a large subset of graphs, we show that the formula is accurate for weak coupling. An approximate gap equation is derived whose solutions describe the theory near the critical point.