  Deep spectrophotometry has proved to be a fundamental tool to improve our
knowledge on the chemical content of planetary nebulae. With the arrival of
very efficient spectrographs installed in the largest ground-based telescopes,
outstanding spectra have been obtained. These data are essential to constrain
state-of-the-art nucleosynthesis models in asymptotic giant branch stars and,
in general, to understand the chemical evolution of our Galaxy. In this paper
we review the last advances on the chemical composition of the ionized gas in
planetary nebulae based on faint emission lines observed through very deep
spectrophotometric data.
