This dissertation deals with the phytochemical and biological investigations of Grindelia tarapacana Phil. (Asteraceae), a plant species native to the Desert of Atacama in Chile. Ten compounds were isolated by using various chromatographic techniques. Of these, seven are new and two are known diterpenoids of the manoyloxide type. One known steroid was also characterized during the course of this study. New diterpenoids included 14S,15-dihydroxy-13-epi-manoyloxide (tarapacol), 15-acetoxy-14S-hydroxy-13-epi-manoyloxide (tarapacol 15-acetate), 14S,15-diacetoxy-13-epi-manoyloxide (tarapacol diacetate), 11α,14S,15-trihydroxy-13-epi-manoyloxide (tarapacanol A), 14S,15-diacetoxy-11α-hydroxy-13-epi-manoyloxide (tarapacanol A 14, 15-diacetate), 12α,14S,15-trihydroxy-13-epi-manoyloxide (tarapacanol B) and 14S,15-dihydroxy-11-keto-13-epi-manoyloxide (tarapacanone). The chemical structures and stereochemistry were established on the basis of extensive spectral analyses including 2D NMR and NOE techniques. X-ray diffraction analysis of tarapacol 15-acetate supported its absolute configuration. The configurations of the other new remaining diterpenoids were assigned based on biogenetic considerations. The two known diterpenoids were 13-epi-manoyloxide and 12α-hydroxy-13-epi-manoyloxide. The steroid was identified as α-spinasterol. The characterization of the known compounds was based on the comparisons of their spectral and physical constants with those reported in the literature for standard samples. Ten known flavonoids were also identified. As part of screening studies for biological activity, anti-HIV and anti-Mycobacterium tuberculosis tests were carried out for the isolated compounds. Five diterpenoids were found to exhibit biological activities. In an anti-HIV test, 12α-hydroxy-13-epi-manoyloxide (at 31 μg/mL) strongly decreased the HIV antigen release to a 10% level and still kept the 84% cell survival, suggesting anti-HIV activity with high selectivity in vitro. The activity of several diterpenoids against Mycobacterium tuberculosis reference strain H₃₇ Ra in vitro was very positive. Three diterpenoids, tarapacol (MIC = 32 μg/mL), tarapacol 15-acetate (MIC = 32 μg/mL) and tarapacanol A 14,15-diacetate (MIC = 32 μg/mL) showed a potency similar to that obtained for the anti-tuberculosis agent pyrazinamide (MIC = 40 μg/mL). Tarapacol diacetate (MIC = 16 μg/mL) was found to be much more potent than pyrazinamide.