The taxonomy of Medicago has been complicated and confused; this monograph provides a clear classification, reducing the number of species to 87 while transferring some previously from Trigonella to Medicago. The great economic importance of the genus, especially M. sativa (alfalfa) is described briefly. The biogeography, chromosome evolution and interbreeding barriers of the species are described, as well as their adaptations and co-evolution with nitrogen-fixing bacteria. Phylogenetic lineages are discussed on the basis of both morphological characters and nucleotide sequence analyses; tables of hypothesised relationships of species are based on morphology, ribosomal internal transcribed spacers and genes. Provisional sections (14) and subsections are given in the light of nucleic acid sequence analyses. Identification of species and inter-specific taxa is dealt with in detail: a table of characters distinguishing species is given and there is a detailed key. Accounts of the species comprise the bulk of the text. Alfalfa is treated at length and those annuals brought into cultivation recently also get detailed coverage. Brief notes on agronomy, pests and diseases are given where appropriate, but the notes concentrate on names, authorities, distribution, identification, habitat, chromosome number and genetic relationships. The text is clear and concise; there is a copious bibliography. This book should be in the libraries of institutions dealing with agronomy, ecology and taxonomy.