Generalized perturbation methods relevant to quantities linear with the neutron flux in time-dependent problems are discussed. In addition, it is shown how the well-known generalized perturbation formulations relevant to the static case under critical conditions can be simply derived. The time-dependent formulations in the neutron field may as well he applied to that of the nuclides in burnup and buildup problems by considering the buildup and decay matrix in place of the Boltzmann operator.