Abstract
In this first part (I) we derive some général formulae for Mössbauer transmission and scattering in the absence of relaxation. Emphasis is put on the influence of the linewidth of the incident radiation on physical observables. We also examine the angular selection rules for scattering by a powder. In the second part (II) we will introduce relaxation and compare the advantages and limitations of the different Mössbauer techniques in relaxation studies