  We compute the one-loop Casimir energy of gravity and matter fields, obeying
various boundary conditions, in 5-dimensional S^1/Z_2 and 6-dimensional T^2/Z_k
orbifolds. We discuss the role of the Casimir energy in possible radius
stabilization mechanisms and show that the presence of massive as well as
massless fields can lead to minima with zero cosmological constant. In the 5-d
orbifold, we also consider the case where kinetic terms localized at the fixed
points are not small. We take into account their contribution to the Casimir
energy and show that localized kinetic terms can also provide a mechanism for
radius stabilization. We apply our results to a recently proposed 5-dimensional
supersymmetric model of electroweak symmetry breaking and show that the Casimir
energy with the minimal matter content is repulsive. Stabilizing the radius
with zero cosmological constant requires, in this context, adding positive bulk
cosmological constant and negative brane-tension counterterms.
