TAB in its initial stage is a compilation of over 100 invited chapters, each reviewing in detail an important and interesting aspect of the plant Arabidopsis thaliana, with reference to what is known in other plants and in other kingdoms. Each set of authors has agreed that their chapter can be kept up-to-date either by the original authors or by others. TAB will therefore be a dynamic information resource that will evolve with the state of knowledge. We envision evolution of the format as well as the contents - initially by taking advantage of the Web-based possibilities of hyperlinks, which will be added to cross-reference gene names and references to other chapters, and which will eventually lead from each gene and sequence reference to sequence databases, and from each paper cited to its abstract. The Arabidopsis Book is therefore envisioned as a complementary resource to the many databases that contain specific information about Arabidopsis.