Numerical taxonomic studies were conducted on 164 species, including all of Medicago and Melilotus and most of Trigonella, the three genera of subtribe Trigonellinae, in order to clarify generic circumscription. The suite of 54 traditionally employed characters studied proved to be polythetic for all groupings of interest, i.e., no subset of individually fully discriminating characters was present, although the suite could be used collectively for discrimination. Medicago proved to be quite distinctive from Melilotus, but not from Trigonella. It was recently found that 51 species traditionally placed in Medicago and 32 species commonly excluded from this genus (and often placed in Trigonella) shared without exception a number of chemical, floral and seed characters, not studied here. That is, all of these 83 species can be assigned to Medicago, which is accordingly defined monothetically. In the present study it was found that characters used traditionally to circumscribe Medicago (with 51 species) by general morphological trends (i.e., polythetically) can also be used, although somewhat less effectively, to circumscribe Medicago in the broad sense (with 83 species). Therefore, prior to the recent discovery of characters that define Medicago monothetically, this genus was defineable only by general morphological trends, and these trends favored the narrower circumscription rather than the broader one now known to be justified. Variation between Trigonella and Melilotus also proved to be gradualistic for the characters studied. Unlike the Medicago–Trigonella problem, however, there are presently no known sets of individually diagnostic characters by which the continuum of variation should be split. These two genera are (arbitrarily) distinguishable as presently circumscribed, only tradition preventing the adoption of alternative, equally defensible ways of segregating them, or indeed of combining them. The study of Trigonella produced groupings essentially identical to the 12 sections of the classical monographer, Sirjaev, and clarified the relationships of several species. The study of Melilotus separated annuals and perennials, corresponding to the subgenera of the classical treatment of Schulz, but failed to reflect his sectional taxonomy. Melilotus bicolor is much closer to what is presently conceived of as Trigonella than to Melilotus.