  Let
  $$L_0=\suml_{j=1}^nM_j^0D_j+M_0^0,\,\,\,\,D_j=\frac{1}{i}\frac{\pa}{\paxj},
  \quad x\in\Rn,$$
  be a constant coefficient
  first-order partial differential system, where the matrices $M_j^0$ are
Hermitian. It is assumed that the homogeneous part is strongly propagative. In
the nonhomegeneous case it is assumed that the operator is isotropic . The
spectral theory of such systems and their potential perturbations is expounded,
and a Limiting Absorption Principle is obtained up to thresholds. Special
attention is given to a detailed study of the Dirac and Maxwell operators.
  The estimates of the spectral derivative near the thresholds are based on
detailed trace estimates on the slowness surfaces. Two applications of these
estimates are presented:
  \begin{itemize}
  \item Global spacetime estimates of the associated evolution unitary groups,
that are also commonly viewed as decay estimates. In particular the Dirac and
Maxwell systems are explicitly treated.
  \item The finiteness of the eigenvalues (in the spectral gap) of the
perturbed Dirac operator is studied, under suitable decay assumptions on the
potential perturbation. \end{itemize}
