  High-precision spectroscopy of large stellar samples plays a crucial role for
several topical issues in astrophysics. Examples include studying the chemical
structure and evolution of the Milky Way galaxy, tracing the origin of chemical
elements, and characterizing planetary host stars. Data are accumulating from
instruments that obtain high-quality spectra of stars in the ultraviolet,
optical and infrared wavelength regions on a routine basis. These instruments
are located at ground-based 2- to 10-m class telescopes around the world, in
addition to the spectrographs with unique capabilities available at the Hubble
Space Telescope. The interpretation of these spectra requires high-quality
transition data for numerous species, in particular neutral and singly ionized
atoms, and di- or triatomic molecules. We rely heavily on the continuous
efforts of laboratory astrophysics groups that produce and improve the relevant
experimental and theoretical atomic and molecular data. The compilation of the
best available data is facilitated by databases and electronic infrastructures
such as the NIST Atomic Spectra Database, the VALD database, or the Virtual
Atomic and Molecular Data Centre (VAMDC). We illustrate the current status of
atomic data for optical stellar spectra with the example of the Gaia-ESO Public
Spectroscopic Survey. Data sources for 35 chemical elements were reviewed in an
effort to construct a line list for a homogeneous abundance analysis of up to
100000 stars.
