  We review the occurrence of negative energies in Pais-Uhlenbeck oscillator.
We point out that in the absence of interactions negative energies are not
problematic, neither in the classical nor in the quantized theory. However, in
the presence of interactions that couple positive and negative energy degrees
of freedom the system is unstable, unless the potential is bounded from bellow
and above. We review some approaches in the literature that attempt to avoid
the problem of negative energies in the Pais-Uhlenbeck oscillator.
