  We review the field of exoplanetary biosignatures with a main focus upon
atmospheric gas-phase species. Due to the paucity of data in Earth-like
planetary atmospheres a common approach is to extrapolate knowledge from the
Solar System and Early Earth to Earth-like exoplanets. We therefore review the
main processes (e.g. atmospheric photochemistry and transport) affecting the
most commonly-considered species (e.g. O2, O3, N2O, CH4 etc.) in the context of
the modern Earth, Early Earth, the Solar System and Earth-like exoplanets. We
consider thereby known abiotic sources for these species in the Solar System
and beyond. We also discuss detectability issues related to atmospheric
biosignature spectra such as band strength and uniqueness. Finally, we
summarize current space agency roadmaps related to biosignature science in an
exoplanet context.
