Trees, shrubs, or herbs, sometimes climbing or decumbent, very often bearing root-nodules that harbor nitrogen-fixing bacteria. Leaves alternate or rarely opposite, pinnate or bipinnate, less often palmately compound or 3-foliolate, seldom 1-foliolate or simple, or modified into narrow phyllodes; petiole present or absent; stipules and stipels present or absent, sometimes stipules developed into spines. Flowers bisexual, rarely unisexual, actinomorphic (Mimosoideae), ± zygomorphic (Caesalpinioideae) to very zygomorphic (Papilionoideae), mostly in racemes, corymbs, spikes, heads, or panicles. Sepals (3-)5(or 6), free or connate into a tube, sometimes bilabiate, rarely reduced or obsolete. Petals (0-)5(or 6), usually isomerous with sepals, seldom fewer or none, imbricate or valvate, distinct and often highly differentiated into papilionaceous corolla: upper petal (standard) outermost, 2 lateral petals (wings) ± parallel with each other, lower 2 innermost petals usually connate by their lower margins and forming a keel. Stamens mostly 10, sometimes fewer or more numerous, distinct or often connate by their filaments to form a closed or open sheath, monadelphous or diadelphous, anther 2-locular, opening lengthwise or by pores, uniform or dimorphic and then alternately basifixed and dorsifixed; pollen simple or compound. Gynoecium nearly always of a solitary carpel (rarely 2 or more distinct carpels); ovary superior, 1-locular or sometimes transversely, rarely longitudinally septate; ovules 1 to numerous, inserted on adaxial suture. Legumes dehiscent by one or both sutures, or indehiscent, sometimes winged, sometimes jointed and breaking up into 1-seeded segments. Seeds without or with very scanty endosperm, sometimes strophiolate.
