Loop Structures

We have collected together the observational evidence concerning the known loop structures in the Galaxy. Our data come from radio continuum surveys, from optical observations in Hcc, from an analysis of interstellar polarization and from 21 cm surveys at low and, to some extent, at high velocities. One interpretation of the results is that the typical loop structure is a spherically symmetrical expanding shell within the disk of the Galaxy, and this is the view that we adopt. The typical shell contains a mass of 10 Me, and an amount of kinetic energy of the order of 1051 erg. If the local density of loop structures is representative of their density in the galactic disk as a whole, then their total contribution would, on this interpretation, be extremely im. portant in the energy balance of the interstellar medium. Key words: radio continuum loops - expanding shells - interstellar polarization.