In this review I discuss recent results on the properties of diffuse ionized gas in spiral galaxies. This gas, also referred to as the warm ionized medium, contains most of the mass of the ionized interstellar medium, and fills a much larger fraction of the disk volume (about 20%) than HII regions. It is a major component of the interstellar medium in disk galaxies, and is particularly relevant because of the large amount of energy that is required to keep the medium ionized. I discuss the spatial distribution and morphology of the diffuse ionized medium in disk galaxies, both from an edge-on and face-on perspective, and the kinematic properties, which are linked to the energy input and turbulent support for this gas. The turbulent properties of HII regions are discussed elsewhere in this volume.

One of the important results is that diffuse ionized gas is present in all spiral galaxies, and contributes, to first order, the same fraction of the total Hα luminosity in a galaxy, independent of the Hubble type or star formation rate. A second important result is that this fraction is so high that Lyman continuum photons from OB stars appear to be the only viable source of ionization for the bulk of this medium. Measurements of forbidden line ratios generally agree reasonably well with photo ionization models, but not in all circumstances. Another potential problem with the photo ionization model is the ionization state of helium.