  We investigate the critical behaviour of the $N$-component Euclidean $\lambda
\phi^4$ model at leading order in $\frac{1}{N}$-expansion. We consider it in
three situations: confined between two parallel planes a distance $L$ apart
from one another, confined to an infinitely long cylinder having a square
cross-section of area $A$ and to a cubic box of volume $V$. Taking the mass
term in the form $m_{0}^2=\alpha(T - T_{0})$, we retrieve Ginzburg-Landau
models which are supposed to describe samples of a material undergoing a phase
transition, respectively in the form of a film, a wire and of a grain, whose
bulk transition temperature ($T_{0}$) is known. We obtain equations for the
critical temperature as functions of $L$ (film), $A$ (wire), $V$ (grain) and of
$T_{0}$, and determine the limiting sizes sustaining the transition.
