  Due to their production sites, as well as to how they are processed and
destroyed in stars, the light elements are excellent tools to investigate a
number of crucial issues in modern astrophysics: from stellar structure and
non-standard processes in stellar interiors to age dating of stars; from
pre-main sequence evolution to the star formation histories of young clusters
and associations and to multiple populations in globular clusters; from Big
Bang nucleosynthesis to the formation and chemical enrichment history of the
Milky Way Galaxy, just to cite some relevant examples. In this paper, we focus
on lithium, beryllium, and boron and on carbon, nitrogen, and oxygen. LiBeB are
rare elements, with negligible abundances with respect to hydrogen; on the
contrary, CNO are among the most abundant elements in the Universe. Pioneering
observations of light-element surface abundances in stars started almost 70
years ago and huge progress has been achieved since then. Indeed, for different
reasons, precise measurements of LiBeB and CNO are difficult, even in our Sun;
however, the advent of state-of-the-art ground- and space-based instrumentation
has allowed the determination of high-quality abundances in stars of different
type, belonging to different Galactic populations. Noticeably, the recent large
spectroscopic surveys performed with multifiber spectrographs have yielded
detailed and homogeneous information on the abundances of Li and CNO for
statistically significant samples of stars; this has allowed us to obtain new
results and insights and, at the same time, raise new questions and challenges.
A complete understanding of the light-element patterns and evolution in the
Universe has not been still achieved. Perspectives for further progress will
open up soon thanks to the new generation instrumentation that is under
development and will come online in the coming years.
