My research goal is to bring us closer to an understanding of the structure and evolution of galaxies, one of the outstanding problems in astronomy. Galaxies evolve through the formation and death of stars, and I study both these processes through their connection with the interstellar gas. Stars affect the gas mainly through radiation and mechanical energy in the form of winds and supernovae. These can have dramatic effects on the distribution and kinematics of interstellar gas, often creating thick gas layers or even gas halos around the star-forming disks. One phase of the interstellar gas that is closely tied to these energetic processes is the so-called diffuse ionized gas. I study this gas in edge-on spiral galaxies, where it is easy to study the global distribution and kinematics of the gas and its vertical structure. I also study in more face-on spirals how various gas phases react to spiral density waves, and the consequences for star formation. The compression in a spiral density wave can alter the physical state of the gas, particularly the star-forming molecular component, and in some cases apparently triggers the formation of new stars.