A general framework for the incoherent scatter radar spectral theories is presented in terms of the generalized Nyquist theorem, KramersKronig relations, and the characteristic functions of random charge-carrier displacements in the ionosphere. Specific spectral models for typical ionospheric conditions are derived and discussed. The discussions focus on the effect of Coulomb collisions in magnetized plasmas, which is a topic of current interest treated in further detail in the companion paper by the authors, as well as on different combinations of physical principles that can be invoked in the derivation of incoherent scatter spectral models. © 2006 IEEE.