This thesis describes the synthesis of several classes of benzimidazoles and benzothiazoles in order to investigate their antiproliferative and antioxidant activity. By using multi-step linear synthesis of new benzazole conjugates, classical synthetic approaches as well as some modern synthetic methods, including synthesis in environmentally friendly solvents or microwave-assisted synthesis, were applied. New acrylonitrile derivatives of N-substituted benzazoles 32‒71 and 77‒107 were prepared by aldol condensation of the corresponding cyanomethylbenzazoles with benzaldehydes with a variable number of methoxy and hydroxy groups and 4-N,N-dimethylamino and 4-N,N-diethylamino groups. Benzimidazole derived Schiff bases 117‒132 were prepared by condensation of N-substituted 2-aminobenzimidazoles 108‒115 with corresponding 4-N,N-dimethylamino and 4-N,N-diethylamino-substituted benzaldehydes. Iminocoumarin and coumarine derivatives 134‒149 and amidino-substituted coumarine derivatives 164‒175 obtained by cyclocondensation of substituted 2-hydroxybenzaldehydes with corresponding 2-cyanomethylbenzimidazoles as well as from coumarine aldehydes with corresponding 4-amidino-substituted 1,2-phenylenediamines using p-benzoquinone as an oxidant. Amidino-substituted benzimidazoles 193‒216 were synthesized within the condensation of 5-substituted salicylaldehydes with corresponding 4-amidino-substituted 1,2-phenylenediamines. Amidino-substituted benzothiazoles 179‒181 i 216‒227 were prepared by condensation of differently substituted 2-hydroxybenzaldehydes and corresponding zwitter ions in glacial acetic acid. Methoxy-substituted carboxamides 235‒262 were prepared by condensation of benzoyl chlorides with N-substituted 2-aminobenzimidazole derivatives. Hydroxy-substituted amide derivatives of N-benzimidazole 263‒268 and 280‒286 obtained by removing protective methoxy groups, using BBr3 at low temperatures, and benzyl protective groups by catalytic hydrogenation with Pd/C in methanol. Amidino-substituted benzamide derivatives 289‒293 were synthesized via acid-catalyzed Pinner reaction from the corresponding cyano-substituted starting precursors. Structures of all newly prepared compounds were confirmed by 1H and 13C NMR spectroscopy while some of them were additionally characterized by mass spectrometry. All prepared compounds were tested for their antiproliferative activity in vitro on a number of human cancer cell lines as well as normal fibroblasts, while antioxidant activity in vitro was performed using spectroscopic DPPH, FRAP and ABTS methods. Amidino-substituted derivatives 164‒175 and 179‒181 were tested for antiviral activity in vitro on several virus strains, and some compounds have shown pronounced and selective activity against some viruses. Schiff base derived benzazoles 117‒132 and amidino-substituted benzazoles 193‒216 and 217‒228 were tested for antibacterial activity against Gram positive and Gram negative bacteria. Results obtained from evaluation of biological activity and SAR studies, revealed that the antioxidant activity is affected by the number of methoxy and hydroxy groups on the phenyl ring as well as the substituent on the nitrogen atom of the benzimidazole nucleus. The strongest impact on the enhancement of antiproliferative activity was observed by 4-N,N-diethylamino group placed at the position 7 on coumarin ring and phenyl ring of acrylonitrile derivatives. The isobutyl substituent on the N atom of the benzimidazole core has the greatest influence on increasing the activity of the synthesized compounds. Some of the most active benzazole derivatives were additionally evaluated to study their mechanisms of biological action and it was confirmed that some of the acrylonitrile and iminocoumarin derivatives have proven to be tubuline polymerization inhibitors, while for amide derivatives the antioxidative activity was tested also in the cells. It has been proven that the most promising antiviral activity has been possessed by coumarine derived benzimidazole substituted with amidine group, being inhibitor of an early step in the replication cycle of virus, respectively the synthesis of viral RNA.