  The study of photoionised gas in planetary nebulae (PNe) has played a major
role in the achievement, over the years, of a better understanding of a number
of physical processes, pertinent to a broader range of fields than that of PNe
studies, spanning from atomic physics to stellar evolution theories. Whilst
empirical techniques are routinely employed for the analysis of the emission
line spectra of these objects, the accurate interpretation of the observational
data often requires the solution of a set of coupled equations, via the
application of a photoionisation/plasma code. A number of large-scale codes
have been developed since the late sixties, using various analytical or
statistical techniques for the transfer of continuum radiation, mainly under
the assumption of spherical symmetry and a few in 3D. These codes have been
proved to be powerful and in many cases essential tools, but a clear idea of
the underlying physical processes and assumptions is necessary in order to
avoid reaching misleading conclusions. A brief review of the field of
photoionisation today is given here, with emphasis on the recent developments,
including the expansion of the models to the 3D domain. Attention is given to
the identification of new available observational constraints and how these can
used to extract useful information from realistic models. (abridged)
