Quasi Stationary Spiral Structure is the natural Interpretation for grand deqlgn spiral galaxies. The best theoretical tool to descrlbe QSSS morphologles is the study of spiral modes. Modal studies are characterlzed by two distinct stages. At the dynamical level, one wishes to compute spiral modes efflcently on a variety of equilibrium models and to master the relation between the properties of the basic states and the propertles of the relevant spiral modes. At the astrophysical level, realistic basic states are identified by dlscussing the processes of evolutlon and
self-regulatlon; in partlcular, we Identify some basic states conslstent wlth QSSS grand designs and others where more flocculent spiral structure is expected to occur. For thls second stage It is cruclal to have a proper interpretation of the results obtained from one-component models In vlew of the multiple component astrophysical systems to be described (which include stars, Population 1 objects, and cold gas). Followlng this outllne, under the guidance of a slmple analytlc (asymptotic) theory, a very large set of galaxy models has been studled numerically. Thls is essentlally a three dimensional survey wlthln a flexlble class of models, where one parameter measures the amount of partlclpatlng disk mass, a second parameter determlnes the
hotness of the disk and the third is a scale length which combines propertles of the slze of the nuclear bulge and the dlstrlbutlon of the cold gas component. All the essentlal morphologlcal types found in the Hubble diagram have been reproduced; in partlcular, parameter reglmes are identified for SA and SB galaxies and speclfically for SBO, SB-r and SB-s objects. Dlstinction is made between systems where gas is expected to play an actlve or a passlve role.