The dynamics of stars and gas appear markedly different in flocculent, tidally induced and quasi-stationary spiral galaxies. Numerical simulations show that the gas and stars exhibit fairly chaotic dynamics during a tidal interaction, unlike the predictions of density wave theory. For a flocculent galaxy, the stars and gas tend to be situated in potential minima associated with the stellar distribution. I will demonstrate a possible observational test to distinguish between drivers of spiral structure by comparing the ages of stellar clusters, using the results of the simulations as an example.

With high resolution simulations, we are also starting to resolve giant molecular clouds, and can thus relate the properties of these clouds to the presence of spiral structure. Calculations so far suggest that spiral structure simply defines where the clouds form, rather than directly triggering star formation. In the last section of this review, I highlight recent efforts to produce synthetic HI and CO observations from numerical simulations.