Because of its proximity (690kpc), the Andromeda galaxy is a unique opportunity to study the distribution of FIR emission lines in a spiral galaxy: it is the only density-wave galaxy where spiral arms are resolved by ISO (M33 being more stochastic). Physical conditions (excitation, temperature and density), as well as the spectral type of exciting stars, can be determined, as a function of location with respect to spiral arms, or main active regions. Observations of the Milky Way cannot give this global structure across an arm due to our unfavored location in the plane of the galaxy. A wealth of data are available already on M31, allowing fruitful comparisons with tracers of star formation: cold and dense gas through CO maps, atomic and more diffuse gas at 21cm, ionised gas (Halpha), UV and FIR tracing young stars. The CII map will bring new and complementary information in that it is free of extinction, and can probe the star-formation deep into molecular clouds (contrary to UV and Halpha); since it is the main coolant of excited regions, and the product of CO photodissociation, it will trace the precise locations of star-formation activity (and complement IRAS data). We will then determine the role of spiral arms and in general large-scale trigger in star-formation.