In this dissertation, three different medicinal plants from Chile and the Fiji Islands, Baccharis linearis (R. et P.) Pers, Aristotelia chilensis (Mol.) Stuntz, and Dysoxylum lenticellare Gillespie were chemically studied. Fourteen compounds isolated from Baccharis linearis were identified based on IR, NMR and mass spectroscopic methods. These constituents included three new neo-clerodane type diterpenes, baclinal (1), baclinepoxide (2), and 13-epi-baclinepoxide (3), as well as one new perhydroazulene derivative, baclinic acid (6). The other identified compounds included portulide B (4), jewenol A (5), oleanolic acid (7), stigmasta-7, 22-dien-3 β-ol (8), stigmasta-7, 22-dien-3 β-ol β-D-glucopyranoside (9), maslinic acid (10), lachnophyllum ester (11), nepetin (12), quercetin 3-methyl ether (13) and werneria chromene (14). With the exception of oleanolic acid, lachnophyllum ester, and werneria chromene, all the other compounds are reported for this species for the first time. Oleanolic acid was isolated as the major component with a yield of 0.3% of dry plant material. Werneria chromene and lachnophyllum ester displayed anti-Mycobacterium tuberculosis activity as well as activity in the brine shrimp test (BST). Six alkaloids isolated from Aristotelia chilensis were identified as aristoteline (15), aristotelinone (16), serratoline (17), aristone (18), 2-epi-aristotelone (19), and aristotelone (20). Serratoline was previously isolated from Aristotelia serrata, a species native to New Zealand. This is the first study that reports serratoline as a natural constituent of A. chilensis from Chile. Another alkaloid, 2-epi-aristotelone, was previously obtained as a synthetic product and is reported here for the first time as naturally occurring. Aristoteline was isolated as the major compound (370 mg) with a yield of only 0.04% as based on dry biomass. Aristoteline showed a weak activity in the brine shrimp test. This work also led to the isolation and characterization of three biflavonoids from Dysoxylum lenticellare. Two biflavonoids were identified as isoginkgetin (21) and bilobetin (22), two of the active components in extracts of Ginkgo biloba, which are used to increase blood-flow and as vascular dilating agents. The third compound was elucidated as the novel natural product robustaflavone 4', 7"-dimethyl ether (23).