Introduction

The subject of discs is central to most of astrophysics, from the formation and dynamics of planetary systems to the formation of protogalaxies in the early universe. Our meeting this week has been very successful, I feel, in bringing out the connections between disc phenomena of very different types and scale sizes. The planning of sessions has played an important part in bringing this about. I would like to thank the organisers for their careful planning, and for all their efforts in making the meeting a success. It was good that all speakers were given sufficient time explain their ideas.

The Compact Oxford Dictionary defines a disc as a “round flattened part in body, plant etc.” In this spirit, and in my capacity as an observer, I will concentrate in my summary on those discs which are actually observed, and which can be assigned a shape. I will start with the smallest discs and work up in size.

Planetary rings

Smallest but by no means the least interesting are the planetary disc and ring systems which were reviewed by Jack Lissauer and Nicole Borderies. Because these systems are relatively simple they are ideal to test theories of density waves, bending waves, edge phenomena and gaps. The purely dynamical phenomena can be studied without having to worry about such messy problems as changes of state (star formation), interactions of the disc with central jets or stellar winds, and interactions with unseen halos.