“Quality and Quantity: maintaining biological diversity in space and time”—Session II of the 5th Symposium of the International Association for Lichenology, Tartu, Estonia, 16–21 August 2004. As naturalists and especially as lichenologists, we would all agree that lichen diversity is desirable for its own sake. A species-poor forest looks ‘wrong’ to an experienced outdoors-person, and, conversely, a lichen-rich forest is a delight, even if the viewer knows nothing about lichens per se. This is why many people, not just lichen-lovers, agonize over the loss of lichen diversity in our natural landscape.