Published online by Cambridge University Press: 14 August 2015
If we view spiral structure in galaxies as a manifestation of some dynamical process which we wish to understand, then the observer can distinguish two facets of the problems. In the first place he must consider the observable consequences suggested by theoretical developments. In the second place he must endeavour to interpret the available observations in terms of realistic physical models, where such models may require detailed information beyond that required for an understanding of spiral structure as a dynamical phenomenon. We shall consider these two aspects in turn.