Much has been written about the usual evolution of general-interest online communities. Less attention has been paid to the dynamics of more specialized, expert-led forums — and the unique if ultimately destructive role of expertise. Over the past 25 years, I participated in dozens of such groups. The forums ranged from 200 to 200,000 members, have been hosted on a variety of platforms, and have dealt with topics ranging from electronic circuit design, to emergency preparedness, to collectible antiques. In almost every instance, they followed the same trajectory — so today, I’d like to put forward a general lifecycle model for expert-led communities.