  We analyze the particle-like excitations arising in relativistic field
theories in states different than the vacuum. The basic properties
characterizing the quasiparticle propagation are studied using two different
complementary methods. First we introduce a frequency-based approach, wherein
the quasiparticle properties are deduced from the spectral analysis of the
two-point propagators. Second, we put forward a real-time approach, wherein the
quantum state corresponding to the quasiparticle excitation is explicitly
constructed, and the time-evolution is followed. Both methods lead to the same
result: the energy and decay rate of the quasiparticles are determined by the
real and imaginary parts of the retarded self-energy respectively. Both
approaches are compared, on the one hand, with the standard field-theoretic
analysis of particles in the vacuum and, on the other hand, with the
mean-field-based techniques in general backgrounds.
