Practical approaches to species concepts for living organisms. Viral species. Towards a practical species concept for cultivable bacteria. Species in practice: exploring uncultured prokaryote diversity in natural samples. Species problems in eukaryotic algae: a modern perspective. The species concept in lichens. Fungal species in practice: identifying species units in fungi. Practical aspects of the species concept in plants. Cultivated plant diversity and taxonomy. Species of marine invertebrates: a comparison of the biological and phylogenetic species concepts. Nematode species: concepts and identification strategies exemplified by the Longidoridae, Steinernematidae and Heterorhabditidae. Species in insect herbivores and parasitoids - sibling species, host races and biotypes. The species concept in blood-sucking vectors of human diseases. Recognition of parthenogenetic insect species. The species in terrestrial non-insect invertebrates (earthworms, arachnids, myriapods, woodlice and snails). Species concepts in systematics and conservation biology - an ornithological viewpoint. The species in mammals. The ideal species concept - and why we can't get it. A hierarchy of species concepts: the denouement in the saga of the species problem. Index.