In current hierarchical theories, elliptical galaxies represent an advanced stage of the galaxy formation process: they are massive galaxies found in dense environments. They reflect `early' conditions in galaxy formation as well as subsequent merging processes, making the study of their properties particularly interesting.
However, their old stellar populations and complicated orbital structure make many important questions harder to answer for ellipticals than for spirals. This includes cosmological questions about their ages and precise formation mechanism, about the concentration of their dark matter halos, and the segregation of baryonic matter, as well as dynamical questions about the orbit distribution and total angular momentum, to name only a few.
The subject of this review is the halos of elliptical galaxies and how planetary nebulae observations and dynamical modelling can help answer some of these questions. Several other papers in this session elaborate on the topics discussed here.