  The mid- and far-IR spectral ranges are critical windows to characterize the
physical and chemical processes that transform the interstellar gas and dust
into stars and planets. Sources in the earliest phases of star formation and in
the latest stages of stellar evolution release most of their energy at these
wavelengths. Besides, the mid- and far-IR ranges provide key spectral
diagnostics of the gas chemistry (water, light hydrides, organic species ...),
of the prevailing physical conditions (H2, atomic fine structure lines...), and
of the dust mineral and ice composition that can not be observed from
ground-based telescopes. With the launch of JAXA's SPICA telescope,
uninterrupted studies in the mid- and far-IR will be possible since ESA's
Infrared Space Observatory (1995). In particular, SAFARI will provide full
access to the 34-210um waveband through several detector arrays and flexible
observing modes (from broadband photometry to medium resolution spectroscopy
with R~3,000 at 63um), and reaching very high line sensitivities (~10^-19
Wm^-2, 5sigma-1hr) within a large FOV (~2'x2'). Compared to previous far-IR
instruments (ISO/LWS, Akari/FIS, Spitzer/MIPS and Herschel/PACS), SAFARI will
provide a superior way to obtain fully-sampled spectro-images and continuous
SEDs of very faint and extended ISM sources in a wavelength domain not
accessible to JWST or ALMA. The much increased sensitivity of SPICA will allow
us to step forward and reveal not only the chemical complexity in the local
ISM, but also in the extragalactic ISM routinely.
