Comets have inspired awe since prehistoric times, but even today there
are only a few thousand comets with well-determined orbits. Nevertheless, the
analysis of this limited sample yields a compelling model for the formation,
evolution and present distribution of comets. This model implies that the
primary source of comets is the Oort cloud, containing over 100 billion comets
at 5,000 to 50,000 times the Earth-Sun distance. I will review our current
understanding of the structure and formation of the Oort cloud, and what comets
can tell us about possible undiscovered planets beyond Neptune. Finally, I’ll
describe some of the puzzles arising from the recent discovery of the
interstellar asteroid/comet, ʻOumuamua’.