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The interstellar medium (ISM) plays an important role in the formation and evolution of a galaxy. The ISM provides the material to form stars and stars in turn inject radiation, metals, and mechanical energy into the ISM, altering the physical conditions, abundances, and distribution of the ISM and affecting future generations of star formation. It is thus essential that we understand the physical structure of the ISM, the physical processes that operate in the ISM, and the interplay between stars and ISM.

The physical structure and processes of the ISM are best studied in the Galaxy and nearby galaxies, and this starts with large-scale surveys for different components of the ISM. For example, the International Galactic Plane Survey (IGPS) of the HI component presents the backbone of the interstellar structure in the Galactic plane, while the HI Nearby Galaxy Survey (THINGS) maps out the HI in nearby galaxies. Spitzer and Herschel have revealed the dust component at high angular resolution, and the Bolocam Galactic Plane Survey (BGPS) and the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) are mapping out the colder dust in the Galaxy. The Atacama Large Millimeter Array (ALMA) will provide clear view of dust and star formation in the Galaxy and nearby galaxies. The polarimetric capabilities of Institut de Radioastronomie Millimetrique (IRAM) in lines, Planck in continuum, and the upcoming ALMA in mm-submm wavelengths are opening a new window to probe magnetic fields. The Wisconsin H-Alpha Mapper (WHAM) has mapped the distribution and velocities of warm ionized gas in the Galaxy from the north and is extending to the south. Chandra and XMM-Newton Observatories have been used to investigate the distribution and physical properties of the 10^6 K hot ionized gas in star forming regions as well as diffuse fields in the Galaxy, Magellanic Clouds, and nearby galaxies. The Far UV Spectroscopic Explorer (FUSE) and the Hubble Space Telescope (HST) STIS and COS have been used to probe the 10^5 K hot gas at interfaces and in the Galactic halo. The Fermi Gamma-Ray Observatory has resolved the gamma-ray emission and revealed the sites of cosmic-ray production in the Galaxy as well as the Magellanic Clouds, and, most recently, the existence of huge bubbles in the Galactic Center of mysterious origin.

During the past one and a half decades, Considerable progress has been made in the numerical modelling of local and global conditions of the ISM, its morphology and its time-dependent evolution, owing to a rapid development of suitable hard- and software. It is now possible to follow the full non-linear evolution of a plasma by solving the hydro- or MHD equations in high resolution simulations with adaptive mesh refinement. One of the key results of the past years was to recognize, and quantitatively describe, the role of compressible turbulence in the ISM and its impact on the distribution of gas into phases, on the mixing of chemically enriched material, on the volume and mass filling factors of the ISM plasma, on its heating and cooling history, amongst others. However, there is still a long way to go, as the requirements of resolution and evolution time are extremely challenging, and analytical models are needed to help us understand complex numerical results. Thus, it is time to review where we stand, and to examine further steps for the future that would lead to a meaningful interpretation of the observations.

A great deal has already been learned from the recent advances in observations and theories. Most meetings have been organized to focus on specialized topics or missions, for example, "Stormy Cosmos: The Evolving ISM from Spitzer to Herschel and Beyond" held at Pasadena in November 2010. There has not been a general meeting for the ISM as a whole in the last 15 years. The most "recent" general ISM meetings were held in 1995 "Physics of the ISM and IGM" and "The Interplay between Massive Star Formation, the ISM, and Galaxy Evolution" and in 2002 "From Observations to Self-Consistent Modeling of the ISM in Galaxies". None of these were IAU meetings. It is time to hold an IAU Symposium to update people on recent advances in the ISM observations and theories, discuss a comprehensive picture of the ISM in a galaxy, connect the ISM in the Galaxy to those of nearby galaxies and even high-z galaxies, especially since recent cosmological simulations have shown the importance of feedback mechanisms on the cluster IGM and galaxy ISM scale. Our proposed IAU Symposium to be held at the IAU GA meeting will maximize the participation of both ISM and extragalactic researchers and stimulate discussions across disciplines.

Another important goal of our proposed symposium is to produce a proceeding volume containing a comprehensive set of proceedings papers that can adequately represent our current understanding of the ISM and be used as a desk reference or a textbook. Note that a concurrent symposium proposal "Molecular Gas, Dust, and Star Formation in Galaxies" has been submitted by Drs. Martin Bureau and Yasuo Fukui. Our proposed symposium emphasizes the physical structure/conditions and dynamical processes of the ISM in general, including all phases, both observationally and theoretically, while their proposed symposium emphasizes the cold phases of the ISM and the star formation process. These two symposia will cover very different topics, but are complementary to each other and are proposed to be held in two consecutive weeks during the IAU GA in Beijing in August 2012. It is likely that most of those attending our symposium will attend their symposium, and vice versa.

Finally, we would like to dedicate this symposium to the late Professor John Dyson, the president of Div VI in 2003-2006. John has pioneered the dynamical interactions of stellar winds and outflows with the interstellar gas. He authored the book "The Physics of the Interstellar Medium", which is widely used as a textbook for ISM courses. His enthusiasm and vision has guided many of us in the study of interstellar dynamics. He is sorely missed by all of us.