Introduction

The recognition that astrophysical discs exist was a major intellectual achievement. As Lissauer stressed at this meeting, it was more than 40 years after Galileo discovered peculiar appendages to Saturn (‘two servants for the old man, who help him to walk and never leave his side’) before Huygens published, as an anagram, the first correct model of the Saturn system (‘it is surrounded by a thin flat ring, nowhere touching, and inclined to the ecliptic’). The long delay was due in part to the limited angular resolution of the available telescopes, but also reflects the leap of imagination needed to grasp the true nature of the first known non-spherical celestial body.

Compared with this one example of an astrophysical disc known for over 300 years, the number and variety of discs that have been discovered or inferred in just the last 30 years is remarkable: (1) Saturn's rings have been joined by lesser ring systems around the other three giant planets, all discovered since 1977; (2) there is recent strong evidence that discs are associated with many protostars and young stars (reviewed by Snell), as well as with active galactic nuclei (reviewed by Malkan); (3) it was only in the late 1960's that accretion discs were recognized to be a central ingredient of many close binary star systems, in particular cataclysmic variables and many Galactic X-ray sources; (4) although it has long been known that the solar system formed from a disc, the analysis of realistic models of protoplanetary discs, and direct observations of similar discs (e. g. the β Pictoris disc), began only in the last few years; (5) it is likely that discs play a crucial role in collimating the jets discovered in double radio sources, SS433, and bipolar flows from young stars.