Tomography $\parbox{10cm}{ \emph{ \lq\lq Far better an approximate answer to the ri... ...estion,\\ which can always be made precise.'' -- John W. Tukey}\\ \bigskip }$ This chapter presents an introduction to ground-based tomography of ionospheric emissions. An investigation of the resolution and error sensitivity of a two-dimensional tomographic system is preformed and it is found that the full width at half maximum of the point spread function varies from 4 km at 120 km altitude to 10 km at 210 km altitude for an input point with side lengths of 2 km. Together with the error sensitivity of the problem this implies that the result of the tomographic inversion cannot retrieve fine scale internal structures. However, estimates of the spatial distribution of ionospheric emissions which, in general, have a comparatively simple shape, can be accurately obtained. Further a strengthening of the statistical stopping criteria by Veklerov and Llacer (1989a) is presented.