The overall goals of this research were to design and synthesize novel betulinic acid (BA) derivatives and to evaluate their biological activities, establish structure-activity relationships (SAR), and investigate mechanism(s) of action. Three specific goals were identified in this study: 1) establish the detailed SAR of C-3 modified BA analogs for HIV-1 maturation inhibitors and further improve antiviral activity profiles; 2) investigate the impacts of C-19 isopropenyl moiety to the antiviral activity and develop 28,30-disubstituted BA analogs as novel HIV-1 entry inhibitors; 3) develop 3,28-disubstituted BA analogs as bifunctional HIV-1 inhibitors. More than 50 BA derivatives were synthesized and evaluated in HIV-1 replication inhibition assay in this research. Compound 82 with enlarged C-3' substitution of C-3 side chain showed an extremely potent anti-HIV-1 activity with an EC50 value of 0.0006 [mu]M, which was better than the current clinical trial candidates bevirimat (DSB) in HIV-1IIIB infected MT-2 cell line. In contrast, C-30 substitutions through ether bond do not influence the antiviral potency of the derivatives significantly. Incorporation of water-solubilizing moieties into C-30 position can improve the hydropholicity and solubility of the C-28 modified BA derived HIV-1 entry inhibitors significantly, which led to the discovery of 112 with a significant anti-HIV-1 activity (EC50: 0.09 [mu]M) similar to the previous best hints. The C-28 side chain was further modified to increase the metabolic stability, resulting in the identification of novel 3,28-disubstituted BA analogs 131 and 132, with better stability and extremely potent antiviral activities (EC50 ~0.006 [mu]M) which are slightly better than that of bevirimat. In a different project, we discovered that C-3 modification and C-30 modification can increase the anti-proteasome activity of BA dramatically. 3,30-disubstituted BA analogs may be developed into potent proteasome inhibitors and potential anti-cancer agents, which represents another promising direction for the development of BA derivatives.