The work embodied in this dissertation is mainly concerned with the isolation and characterization of natural chemical constituents having some biological importance. The isolated compounds were either new or previously been reported in the literature and they were characterized by various sophisticated spectroscopic techniques. The dissertation deals with purification and biological screening of secondary metabolites isolated from three indigenous medicinal plants of Pakistan namely Launaea nudicaulis, Launaea intybacea and Carissa opaca. The whole dissertation is divided into six chapters. The chapter 1 entitled as “Human Health and Role of Natural products” describes significance of terrestrial plants in Human life for the cure of various diseases, some FDA approved natural product drugs, natural products under clinical trials for FDA approval and new challenges in the field of natural products research. The second chapter entitled as “The Genus Launaea and its Importance” describes the genus Launaea, its economical, pharmacological and phytochemical properties. In this chapter the pharmacology of various isolates from the genus Launaea and biological activities of various species v or their extract has been discussed. Almost all secondary metabolites isolated up to date from the different species of Launaea have been described year wise. The third chapter entitled as “Phytochemical Investigations on Launaea nudicaulis” deals with the phytochemical studies on Launaea nudicaulis collected from Cholistan desert covers the isolation, structure elucidation and experimental detail of eight new and same number of known secondary metabolites. The methanolic extract of this plant was divided into n-hexane and ethyl acetate fractions. The n-hexane soluble part yielded four new sphingolipids: nudicualin A (112), nudicualin B (113), nudicualin D (114), nudicualin C (115) together with known compounds, 1-hexatriacontanol (109), elaidic acid (110), oleanolic acid (111) and β-sitosterol glucoside (67). The column chromatography of ethyl acetate part yielded four new compounds: a sesquiterpene lactone, nudicholoid (116), a diacetylene derivative, trideca-12-ene-4,6-diyne 2,8,9,10,11-pentaol (119), quinic acid derivative,cholistaquinate (121) and a flavonoid di-C-glycoside, cholistaflaside (123) along with four known compounds, ergosta-7,22-diene-3,5,6-triol (117), benzyl glucopyranoside (118), 3,7,12-trihydroxycholan-24-oic acid (120) and 20-hydroxyecdysone (122). Their structures were assigned by ID and 2D-NMR in combination with EIMS, HR-MS and FAB-MS techniques. Structures of the known compounds were determined by spectroscopic analyses and in comparison with the literature.