The subject matter of the present dissertation deals with isolation, characterization and evaluation of biological activities of selected species belonging to families Solanaceae, Guttiferae and Pinaceae. The enclosed research data of the thesis is divided into following parts. PART A: Phytochemical Studies of the Selected Species of Family Solonaceae PART B: Phytochemical Studies of the Selected Species of Family Guttiferae PART C: Phytochemical Studies of the Selected Species of Family Pinaceae PART D: Evaluation of Biological Activities PART A Part A describes the phytochemical investigation on Witahinia coagulans and Physalis divericata (Solanaceae). Six new (45-50) and ten known (51-60) withanolides have been isolated from W. coagulans of Pakistani origin, whereas withaphysanolide A (61), a novel withanolide together with five known physalins (62-66) and four withaphysalins (67-70) were isolated from the P. divericata. Various experimental techniques and extensive spectroscopic studies were used for the structural elucidation of the compounds. The isolated withanolides were evaluated for inhibition activity on lipopoly-saccharide (LPS) induced B-cell, concanavalin A (ConA)-induced T-cell proliferation and against human colorectal carcinoma HCT-116 and human lung cancer NCI-H460 cells PART B Part B includes the isolation and characterization of constituents from Hypericum species (Guttiferae). Six new (105-110) and fourteen known xanthones (111-124) along with nine other compounds (125-133) have been isolated from H. oblongifolium, while six known compounds (134-139) were isolated from H. dyeri. These components were evaluated for respiratory burst inhibitory (anti-inflammatory), enzyme inhibitory and antioxidant activities. PART C Part C contains the the GC and GC-MS analysis of various extracts from conifers belonging to family Pinaceae. The amount and composition of lipophilic and hydrophilic extractives as well as proanthocyanidins in the bark of seven Pakistani conifers were analyzed. The bioactive polyphenols and other known compounds were found interesting in order to find a potential value-added use of local tree species. Gravimetrically these extracts were analysed for lipophilic and hydrophilic extractactives. The predominant lipophilic extractives were common fatty and resin acids, fatty alcohols, and sterols. Different known lignans, stilbenes, ferulates, and flavonoids were generally predominant among the hydrophilic extractives. Pinus species e.g. P. wallichiana, P. gerardiana and Picea smithiana showed large amounts of lipophilic and hydrophilic extractives compared to the other examined conifers. Pinus roxburghii was found different from the other pine species having smaller amounts of both types of extractives. A. pindrow and T. fuana were also found to have the smallest amount of hexane extracts. The proanthocyanidin content and composition revealed that especially Pinus wallichiana and Abies pindrow could be rich sources of such compounds. PART D Part D is concerned with evaluation of biological activities of crude extracts, fractions, semi-pure and pure constituents. Different solvents soluble fractions of the selected plants belonging to family Guttiferae (H. perforatum, H. oblongifolium, H. monogynum, H. choisianum and H. dyeri), Pinaceae (bark and knotwood of Picea smithiana, Abies pindrow, Pinus wallichiana, P. geradiana, P. roxburghii and Cedrus deodara) and Taxus fauna from the north west of Pakistan were screened for their possible antioxidant activity. Anticancer (anti-proliferative) and enzyme inhibitory activities of Hypericum species as well as the cytotoxic, anti-inflammatory and urease inhibitory activities of pure compounds isolated from Hypericum, Physalis and Withania species were also studied. Four complementary test systems, namely phenolic compounds, free-radical scavenging capacity, measuring of reducing power and total antioxidant activities by Phosphomolybdenum method were used for analysis. We report here for the first time the antioxidant and antimicrobial potential of the various extracts and fractions of the listed plants for the first time except Hypericum perforatum which has been the subject of many investigations. The objectives of this study were to explore the biological and medicinal value of the extract/fractions of the above mentioned plants.