  The Casimir effect giving rise to an attractive or repulsive force between
the configuration boundaries that confine the massless scalar field is
reexamined for one to three-dimensional pistons in this paper. Especially, we
consider Casimir pistons with hybrid boundary conditions, where the boundary
condition on the piston is Neumann and those on other surfaces are Dirichlet.
We show that the Casimir force on the piston is always repulsive, in contrast
with the same problem where the boundary conditions are Dirichlet on all
surfaces.
