Of the many topics in astronomy Gérard de Vaucouleurs has worked in over the past 40 years, I think the one which brought him some of the greatest enjoyment and personal satisfaction was his research on galaxy morphology and classification. Not only is the beauty of galaxies enjoyable from an aesthetic viewpoint, but galaxy morphology is rich with the enigmas and puzzles that qualify it as one of the major research topics of today. Although for many years, there was so little firm theoretical understanding of galaxy morphology that the topic was little more than descriptive, this is certainly no longer true. Galaxy morphology today is, in fact, undergoing a major revolution. Not only has the Hubble Sequence been revitalized as a tool for understanding galaxy structure and evolution, but the many additional phenomena, such as bars and rings, that used to be regarded as secondary, have also been brought to the forefront. Never before, I believe, have the many types of structures visible in galaxy images been understood as well as today, although much remains to be learned.
It is not hard to summarize the main problems of galaxy morphology. Why is the Hubble sequence a continuous sequence, and what physical parameters underlie this continuity? What is the role of angular momentum, dissipation, and galaxy-galaxy interactions? How do the various patterns originate, and how do they change with time? What types of objects define the patterns? What role do the patterns have on the star formation processes in the disk? What is the role of dark matter? These questions have all been asked but definitive answers have been elusive. Ultimately, the answers to these questions will hinge on an understanding of galaxy formation, a topic which is currently a subject of considerable uncertainty and debate.
In honor of Gérard de Vaucouleurs, I should like to briefly describe his 1959 galaxy classification system, and then focus on a few aspects of galaxy morphology that have received much attention over the last few years. Comprehensive reviews on this subject have already been given by Sandage (1975) and Kormendy (1982). I will concentrate mainly on ring and lens morphologies in normal disk galaxies (S0 to Sbc), since these features were of great interest to Gérard and exemplify how much our understanding has progressed about galaxy morphology as a whole. Very comprehensive discussions of elliptical galaxies can be found in I. A. U. Symposium No. 127 (de Zeeuw 1987) and other articles in the present volume, while a thorough review of irregulars can be found in Gallagher and Hunter (1984, 1987).