[Fe II] forbidden lines are among the brightest emission lines in NIR
band. They have been observed toward diverse types of astronomical
sources, from nearby star-forming regions to active galactic nuclei.
These NIR [Fe II] lines are particularly strong in shock-heated dense
atomic gas, which makes them a shock tracer. Numerous emission lines
from the transitions among 16 ground levels of Fe II provide useful
diagnostics of physical conditions in shocked gas. In this talk, I
will first review the physics of NIR [Fe II] forbidden lines, and then
I will present some astrophysical applications regarding interstellar
shocks, supernova remnants, and starburst galaxies.