The study of lithium and beryllium abundances is a topic that covers a
very broad range of astrophysical problems from internal processes in
stars to Galactic evolution and primordial nucleosynthesis.
I will first provide an overview of the use of Li and Be measurements in
stars belonging to different populations as a key tool to address this
variety of topics. Then, I will focus on two different, but related
issues: namely, the use of Li and Be abundances as internal mixing
tracers; and ii) the determination of the age of young Pop. I and old Pop.
II stars through Li and Be measurements, respectively. In this context,
recent results based on open and globular clusters observations will be
presented and discussed.