  The Casimir effect is investigated in light-cone quantization. It is shown
that for spacelike separation of the walls enclosing the system the standard
result for the pressure exerted on the walls is obtained. For walls separated
in light-cone space direction no regularization of the quantum fluctuations
exists which would yield a finite pressure. The origin of this failure and its
implications for other vacuum properties are discussed by analyzing the Casimir
effect as seen from a moving observer approaching the speed of light. The
possibility for calculation of thermodynamic quantities in light-cone
quantization via the Casimir effect is pointed out.
