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Although it comprises only a small fraction of the mass in our galaxy, the interstellar medium fills the vast majority of the volume of our galaxy and plays an important role in the galaxy's evolution. The interstellar medium serves as a repository for the energy, momentum, metals, and mass-loss associated with stellar life and death while providing the source of matter for new generations of stars yet to be born. The processes which govern the stellar birth-life-death-birth cycle and the possible inter-relations and feedback mechanisms between these processes lie at the heart of a galaxy's evolutionary story.
Over the past three decades, observational studies of the nature of the interstellar medium distributed away from the galactic plane have shown that gas can be found several kiloparsecs from the disk. Again, although the fraction of the interstellar medium found in the halo is small, it is thought that it may play an important role in the medium's evolution. The data show that several different gas phases, possessing a wide range of temperatures, densities, ionization levels, scale heights and filling factors, co-exist in the halo. Observations of these and other ``global" properties of the medium, such as its kinematics and pressure, have been described by numerous theories, which can be broadly classified as either thermal or non-thermal in nature. One of the most widely discussed theories, the galactic fountain theory, suggests that the material in the halo is supplied by supernova-heated gas which rises to a large scale height; the variety of observed phases arise from regions of gas in various stages of cooling.
In a different approach, rather than determining the global properties of the halo interstellar medium, recent work has focused on determining and studying specific sites of the transport of matter to and/or from the halo. Using a variety of observational techniques at different wavelengths, efforts have been made to assess the morphology, ionization, abundance and depletion of gas with distinctive z-motions in order to determine if their properties are consistent with galactic fountain or other hypotheses.
I will review the observational bases for our current understanding of the nature of halo gas in both the Milky Way and external galaxies, and discuss the relationship between interstellar disk and halo processes and their impact on the evolution of galactic systems as a whole.
