  Stars like our Sun and planets like our Earth form in dense regions within
interstellar molecular clouds, called pre-stellar cores (PSCs). PSCs provide
the initial conditions in the process of star and planet formation. In the past
15 years, detailed observations of (low-mass) PSCs in nearby molecular cloud
complexes have allowed us to find that they are cold (T < 10 K) and quiescent
(molecular line widths are close to thermal), with a chemistry profoundly
affected by molecular freeze-out onto dust grains. In these conditions,
deuterated molecules flourish, becoming the best tools to unveil the PSC
physical and chemical structure. Despite their apparent simplicity, PSCs still
offer puzzles to solve and they are far from being completely understood. For
example, what is happening to the gas and dust in their nuclei (the future
stellar cradles) is still a mystery that awaits for ALMA. Other important
questions are: how do different environments and external conditions affect the
PSC physical/chemical structure? Are PSCs in high-mass star forming regions
similar to the well-known low-mass PSCs? Here I review observational and
theoretical work on PSCs in nearby molecular cloud complexes and the ongoing
search and study of massive PSCs embedded in infrared dark clouds (IRDCs),
which host the initial conditions for stellar cluster and high-mass star
formation.
