We consider pseudo-unitary quantum systems and discuss various properties of
pseudo-unitary operators. In particular we prove a characterization theorem for
block-diagonalizable pseudo-unitary operators with finite-dimensional diagonal
blocks. Furthermore, we show that every pseudo-unitary matrix is the
exponential of $i=\sqrt{-1}$ times a pseudo-Hermitian matrix, and determine the
structure of the Lie groups consisting of pseudo-unitary matrices. In
particular, we present a thorough treatment of $2\times 2$ pseudo-unitary
matrices and discuss an example of a quantum system with a $2\times 2$
pseudo-unitary dynamical group. As other applications of our general results we
give a proof of the spectral theorem for symplectic transformations of
classical mechanics, demonstrate the coincidence of the symplectic group
$Sp(2n)$ with the real subgroup of a matrix group that is isomorphic to the
pseudo-unitary group U(n,n), and elaborate on an approach to second
quantization that makes use of the underlying pseudo-unitary dynamical groups.