A year ago, Hale-Bopp, the comet, fascinated amateur stargazers and professional astronomers alike. It may since have vanished from our field of vision, but it was the subject of a conference held on Tenerife, one of the Canary Islands, in February and attended by about 100 scientists. One point on which scientists attending the first international Hale-Bopp conference were agreed was that the comet's brightness was due mainly to the size of its nucleus, which was evidently between 40 and 70 km in diameter.