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-foliate, seldom 1-foliate 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 (Papilonoideae), mostly in racemes, corymbs, spikes, heads, or panicles. Sepal (3-) 5 (or6), free or connate into a tube, sometimes bilabiate, rarely reduced or obsolete, Petals (0-) 5 (or6), usually isomerous with sepals, seldom fewer or none, imbricate or valvate, distinct and often highly differentiated into papilonaceouse corolla; upper petal (Standard) outermost, 2 lateral petals (Wings) parallel with each other, lower 2 innermost petals usually connatedBy their lower margins and forming a keel. Stamen 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; ovule 1 to numerous, inserted in adaxial suture. Legume dehiscent by one or both sutures, or indehiscent, sometimes winged, sometimes jointed and breaking up into 1-seeded segements. Seed without or with very scantly endosperm, sometimes strophiolate.