  We discuss the generalization of the Dirac equations and spinors in momentum
space to free unstable spin-$1/2$ fermions taking into account the fundamental
requirement of Lorentz covariance. We derive the generalized adjoint Dirac
equations and spinors, and explain the very simple relation that exists, in our
formulation, between the unstable and stable cases. As an application of the
generalized spinors, we evaluate the probability density. We also discuss the
behavior of the generalized Dirac equations under time reversal.
