The use of natural products with therapeutic properties is as ancient as human civilization and, for a long time, mineral, plant and animal products were the main sources of drugs. Natural products have a vital role in drug discovery and development. Biomolecules derived from natural sources have been used to cure numerous diseases since the emergence of medicine. People used natural products as home remedies to cure many diseases and the folk medicinal system kept transferring from generation to generation. Even in the modern scientific era, secondary metabolites from plants have been revolutionizing the drug development to fight diseases during the 20th century. Phytochemical studies of secondary metabolites from plants have made the foundations to connect the traditional folk medical knowledge to drug development by finding and characterizing the bioactive constituents. Olea ferruginea is an indigenous medicinal plant which is locally called Kao or Khoona and has been found abundantly in the Abbottabad region of Northern Pakistan. It has been used traditionally to treat fever, diabetes, asthma, toothache, rheumatism, malaria, and wounds. Owing to its great medicinal importance, abundant availability, and no previous phytochemical reports, we decided to investigate it phytochemically and pharmacologically. The present thesis describes the isolation and structure elucidation of five new compounds including a secoiridoid glycosidic lignan ester (141), two dimeric secoiridoid glycosides (142, 143), a trimeric secoiridoid (144), and a tetrameric lignan macrocycle (145) whose structures have been established by extensive spectroscopic studies including mass spectrometric techniques, 1H-NMR, 13C-NMR, 1H-1H COSY, HSQC, HMBC, and NOESY experiments. The isolated compounds along with previously isolated compounds by us i.e. quercetin (59), β-amyrin (107), oleuropein (1), and ligstroside (2) have also been tested for various biological activities including anticancer (alkaline phosphatase inhibition and Hela cancer cell lines), aldose reductase (ALR1 and ALR2), α- and β-glucosidase inhibitory activities, antioxidant, and antiglycation activities. The results showed these compounds to be very good anticancer and antidiabetic agents which can serve as leads for new drugs. Computational chemistry has been very helpful in absolute structure determination of complex and conformationally flexible natural molecules by calculating their theoretical NMR, UV, IR, and CD spectra etc. Owing to the great importance of computational chemistry and its applications in organic and natural products, we developed a theoretical model to study the structural properties of flavones using 5,7,3'',5''-tetrahydroxyflavanone (146) as a test case which was previously isolated by us. We also developed a theoretical model to study the geometric and spectroscopic properties of ursane and oleanane triterpenoids based on our studies on representatives of these two classes, i.e. ursolic acid (103) and oleanolic acid (105).