| Abstract: | The [CII] 157.74 μm fine structure transition is one of the brightest and most well-studied emission lines in the far-infrared, produced in the interstellar medium (ISM) of galaxies. We study its properties in sub-pc resolution hydrodynamical simulations for an ISM patch, coupled with time-dependent chemistry, far-ultraviolet dust and gas shielding, star formation, photoionization and supernova feedback, and full line-radiative transfer. We find a [CII]-to-H2 conversion factor that is almost constant with metallicity, and a [CII]-SFR relation that scales linearly with metallicity. The majority of [CII] originates from atomic gas with hydrogen number density n∼10 cm−3. Our findings are consistent with z∼0 observations. As such, [CII] is a good SFR tracer even in metal-poor environments where molecular lines might be undetectable. Resolving the clumpy structure of the dense (n=10−1000 cm^{-3}) ISM is important as it dominates [CII] 157.74 μm emission |