Observations of interstellar gas and dust towards nearby stars and within the solar system show that the Sun is embedded in a warm diffuse partiallyionized cloud. This cloud is the leading edge of a flow of interstellar matter (ISM) through our Galactic neighborhood. Interstellar matter sets the boundary conditions of the heliosphere and astrospheres of extra-solar planetary systems. Moreover, ...

We review the current knowledge and understanding of the interstellar medium of our galaxy. We first present each of the three basic constituents – ordinary matter, cosmic rays, and magnetic fields – of the interstellar medium, laying emphasis on their physical and chemical properties inferred from a broad range of observations. We then position the different interstellar constituents, both wit...