Final Report Summary - SYNISM (The Dust-Gas Synergy in the ISM) Interstellar dust is a fundamental component of the interstellar medium (ISM). By mediating key processes such as the absorption of stellar radiation, which is then re-emitted at infrared-mm wavelengths, it provides the necessary conditions (e.g. cloud cooling) for star formation to occur. Thus, dust determines the very processes that govern galaxy evolution. Dust is a ubiquitous component of the ISM in nearly all types of galaxies, even those at very high redshifts, where the dust has had little time to form. However, dust is an evolving component of the ISM, which responds to the local conditions. If we are to understand the processes that govern galaxy evolution it is therefore essential that we understand the evolving nature of dust in its relationship with the physical and dynamical conditions of the local ISM. The global objective of this proposal has been to investigate the synergy between the dust and the gas in the ISM in the closest available laboratory for such a study - our own Galaxy, and to quantify the factors that drive dust evolution. This project has made use of the latest generation of European telescopes and cutting edge models.Using the unique capabilities provided by Herschel Space Observatory, the researcher has analysed in detail a representative sample of eleven nearby Galactic photo-dissociation regions (PDRs) in the ISM. PDRs are regions of the interstellar medium at the interface between the stars and molecular clouds and are thus ideal as they span a broad range in the physical conditions of galaxies. These transition regions have been mapped with the PACS and SPIRE instruments on board Herschel, operating in the wavelength range from 50 to 600μm. This far-infrared region of the electromagnetic spectrum is home to many important cooling lines that can be used to trace the physical conditions of the gas, and is where the bulk of the dust emission occurs. Observationally, the structures seen in nearby PDRs (e.g. filaments, clumps etc...) are several to tens of arcseconds in size, and are thus ideally suited for Herschel spatial resolution. These observations have enable us to trace the spatial evolution of the physical conditions of the gas across the illuminated interfaces via the study of several emission lines. These local conditions have then been coupled to the dust spectral energy distribution of the same regions to obtain the properties of the grains and trace their evolution.Using radiative transfer models to interpret the emission of the main cooling lines the researcher has been able to trace the variation of the gas local physical conditions as a function of environment in the PDR sample studied PDR. We have correlated them with the emission of dust to understand its evolution. This has provide us with an unprecedented view of how dust evolves from the diffuse to the dense medium where star forms. Finally, the researcher has extrapolated this detail understanding of the nearby ISM to create mid- and far-infrared diagnostic of unresolved galaxies. We have derived key relations between the far-IR line luminosities and infrared luminosities and star formation rates in a sample of luminous infrared galaxies.