Natural drug from the plants are gaining popularity because of several advantages such as often fewer side effect, better patient tolerance, relatively less expensive and acceptance due to a long history of use, especially herbal medicines provide rational means for the treatment of many diseases that are obstinate and incurable in other system of medicine. However, the chemical complexity of many natural products and the lack of assurance of a renewable supply have created a diminishing interest by the pharmaceutical industry, which in turn endorses the pivotal role of academia and public organisations in the protracted exploration and evaluation of natural products( Patwardhan, 2005; Cordell and Colvard, 2005). In the development of "Third Generation" plant based drugs, a top-bottom approach is usually adopted. This consists of first conducting a clinical evaluation of the treatment modalities and therapy as administered by traditional doctors. This evaluation is then followed by acute and chronic toxicity or cytotoxic studies in animals. It is only if the substance has an acceptable safety index would it be necessary to conduct detailed pharmacological or biochemical studies (Smith et al., 1996; Iwu et al., 1999). Formulation and trial production of the dosage forms are structured to mimic the traditional use of the herb. The stability of the finished product is given careful attention during the formulation of the final dosage form. This is a unique blend of the empiricism of the earlier first generation botanicals with the experimental research used to prove the efficacy and safety of second generation isolated pure compounds. Several pharmaceuticals companies are engaged in the development of plant based drugs through the isolation of active molecules from plant extracts (Ramakrishnappa 2003). Therefore, a revival of interest in the use of plants in pharmacy worldwide emerges from both the pharmaceutical industry as a source of new lead molecules and the general public who are using plant extract in many ways in conventional and complementary therapies. Leguminosae is the most significant family in the Dicotyledonae which contains approximately 650 genera and 18000 species. Legumes have prevalent role in world food agriculture. The seeds of legumes are rich in high-class proteins. Numerous plants of this family are used as food, fodders, forages, timber and medicine. Some Legume trees are also excellent source of precious resins and insecticides. Total economic value for leguminous crops is expected to be about two billion US dollars per annum (Estrella et al., 2005). Besides the economic benefits, the plants of this family also produce a array of secondary metabolite that have an outstanding role in medicine e.g. Alkaloids, tannins, glycosides, saponins and terpenoids etc. The role of this family and their species in the present pharmaceutical has been over looked, despite significant evidence that leguminoseous plants in particular, are an important source of medicines for indigenous peoples and have a highly significant over representation in indigenous pharmacopoeias in relation to other types of plants. Some important investigations related to pharmacognostical activity of Acacia and Prosopis species were under taken in this thesis. The aim of these investigations was ethnomedicinal, antibacterial, antifungal, hisotochemical and phytochemical study of selected plants species. The ethnomedicinal study was carried out using a survey, conducted in many rural areas of Rajasthan. In this survey it was found that all the investigated species have an significant role in folk medicine including: Acacia nilotica: barks in toothache and cough; leaves in sexual weakness and diarrhea; seeds in stomach complaint and skin disorder. Acacia tortilis : bark in cough and skin ailments; pods and leaves in indigestion. Acacia senegal: gum in leprosy; aerial parts in cough. Acacia catechu: bark in leucorrhoea and menstrual complaints; leaves in dysentery and gonorrhoea; gum in asthma. Acacia jacquemontii: whole plant used in serve ailment. Prosopis cineraria: flowers mixed with sugar and administered to prevent miscarriage; ashes are rubbed over the skin to remove hair; bark are used in asthma, bronchitis, dysentery, leucoderma, leprosy, muscle tremors, piles and snake bite; leaves are used in eye troubles. The ethnomedicinal uses of above species have also been described by other researchers such as : Acacia nilotica (L.) Del. used in digestive system disorder, syphilis, cholera, dysentery and leprosy; Acacia senegal used in leprosy and inflammation; Acacia catechu used in bronchitis, pain in chest, asthma and cancerous sores (Chopra et al.,1956; Gupta, 1970; Jain and Tarafder, 1970; Joshi, 1982; Das et al., 1983; Katewa, et al., 2004).