Freshwater planarians were a classic model for studying the problems of development and regeneration. However, as attention shifted towards animals with more rigid developmental processes, the planarians, with their notoriously plastic ontogeny, declined in significance as a model system. This trend was exacerbated with the introduction of genetic and molecular approaches, which did not work well in planarians. More recently, the heightened interest in stem-cell biology, along with the successful application of molecular, cellular and genomic approaches in planarians, is re-establishing these fascinating organisms as models for studying regeneration and developmental plasticity.