  A review is presented of the most relevant results obtained in the last few
years on this rare class of astronomical sources. Multi-wavelength analysis of
an increasing number of post-AGB stars reveal that they constitute a more
inhomogeneous population of stars than previously thought. The new data
available allow us to study these sources with unprecedent spatial resolution
and to extend our spectroscopic knowledge in a systematic way to the infrared
for the first time, where crucial information is contained on the chemical
composition of the gas and dust in their circumstellar shells. The overall
infrared properties derived from ISO and Spitzer data can be used to trace the
mass loss history and the chemical evolution of the ejected material. The new
results impose severe observational constraints to the current nucleosynthesis
models and suggest that the evolution is mainly determined not only by the
initial mass but also by the metallicity of the progenitor star. Post-AGB
samples are likely to grow in the near future with the advent of new data from
space facilities like Spitzer or Akari. Studies of post-AGB stars in the
galactic halo, the Magellanic Clouds and other galaxies of the Local Group will
certainly improve our knowledge on the evolutionary connections between AGB
stars and PNe.
