  The Pauli exclusion principle (PEP) represents one of the basic principles of
modern physics and, even if there are no compelling reasons to doubt its
validity, it still spurs a lively debate, because an intuitive, elementary
explanation is still missing, and because of its unique stand among the basic
symmetries of physics. A new limit on the probability that PEP is violated by
electrons was estabilished by the VIP (VIolation of the Pauli exclusion
principle) Collaboration, using the method of searching for PEP forbidden
atomic transitions in copper. The preliminary value, ${1/2}\beta^{2} \textless
4.5\times 10^{-28}$, represents an improvement of about two orders of magnitude
of the previous limit. The goal of VIP is to push this limit at the level of
$10^{-30}$.
