An important feature of many disk galaxies is their spiral structure, which, for the Milky Way, has been connected with star formation since Morgan, Whitford & Code (1953) found concentrations of OB stars in the Sagittarius spiral arm of the Milky Way. This connection suggests that spiral arms trigger star formation, which makes us wonder how such triggering might fit in with the global star formation laws discussed in the previous lecture. The answer is that spiral arms have very little influence on large-scale star formation rates, but they do organize the star formation in a galaxy. This is probably because most of the gas that can turn molecular, i.e., inside shielded regions at normal interstellar pressure, has already done so in the main disks of galaxies, and because this molecular gas is already forming stars as fast as it can. Star formation in this case may be viewed as saturated (Elmegreen 2002). Additional cloud collisions in spiral shocks, or new cloud formation in spiral arms, does not add much to the molecular mass and star formation - it only moves it around. This may not be true in the outer parts of galaxies, where the gas is highly atomic. There, dynamical processes such as spiral arms could affect the average star formation rate. There is very little known about outer disks yet, so the influence of outer spiral arms on average star formation rates remains an open question.
In this lecture, we begin with a description of spiral waves and the various theories for them. Then we discuss detailed models for how spiral arms interact with the gas and affect the formation of giant clouds. We also discuss the interarm clouds and the apparent aging and destruction of dense clouds as they move to the next arm.