This doctoral thesis describes the synthesis, structural characterization and biological activity of new derivatives of 2-arylbenzimidazole and 2-arylbenzothiazole, hydrazone derivatives of benzothiazole and 1,2,3-triazole derivatives of coumarin and iminocoumarin. In addition to conventional synthetic methods, ecologically acceptable green methods were also used, such as synthesis assisted by microwaves, ultrasound and mechanochemical reactions. Ultrasound-assisted cyclocondensation reaction of prepared O-alkylated benzaldehydes (1‒6) or 1,2,3-triazole derivatives of benzaldehyde (10 and 11) with differently substituted 1,2-diaminobenzenes in the presence of Na2S2O5 as an oxidizing agent, gave new O-alkylated (12 ‒27) and 1,2,3-triazole derivatives of 2-arylbenzimidazole (28‒33). 2-(4-alkoxyphenyl)benzothiazole derivatives (47–55) were synthesized conventionally by O-alkylation reaction of prepared 2-(4-hydroxyphenyl)benzothiazole (40–46) with corresponding aminoalkyl halides in the presence of K2CO3, while 2-(4-alkoxyphenyl)-6-halobenzothiazole derivatives (56–67) were prepared by microwave-assisted O-alkylation reaction of 2-(4-hydroxyphenyl)benzothiazole (43–46). Copper-catalyzed Huisgen 1,3-dipolar cycloaddition reaction of morpholine azide (9) and 2-(4-O-(propargylphenyl)benzothiazole derivatives (68–74) gave 1,2,3-triazole derivatives of benzothiazole (75–81). In order to select the synthetic metod of target 2-arylbenzothiazole derivatives substituted at different position of the benzene ring with 1,2,3-triazole ring (88–117), optimization of the reaction conditions was conducted on a model reaction including conventional synthesis, ultrasound-assisted synthesis and mechanochemical synthesis. Since the liquid assisted mechanochemical reactions gave the highest yields of benzothiazole derivatives (88, 98 and 108) with the shortest reaction time, mechanochemistry was selected as the method for the preparation of 2- and 3-O-(1-aryl-1,2,3-triazolyl))phenylbenzothiazole (88‒117) and 2-(4-O-(1-aryl-1,2,3-triazolyl)phenyl)benzothiazole derivatives (118‒156). Hybrids of benzothiazole and 4-alkylaminobenzene bridged by a hydrazone moiety (169–202) were prepared as an E-isomers using solvent-free mechanochemical synthesis by condensation reaction of 4-alkoxybenzaldehydes (1, 2, 4, 5, 161‒168) and 2-hydrazinylbenzothiazoles (158–160). 1,2,3-triazole iminocoumarin (207–212) and coumarin derivatives (213–223) were synthesized by copper-catalyzed microwave-assisted click reaction of the corresponding azides and terminal alkynes. The structures of the prepared derivatives were confirmed by 1H- and 13C-NMR spectroscopy as well as by two-dimensional methods NOESY, HSQC and HMBC. The antiproliferative activity in vitro of the prepared compounds was evaluated against a range human tumor cell lines as well as on healthy cells. Antibacterial activity in vitro was performed on Gram-positive and Gram-negative bacterial strains including clinically resistant strains of MRSA and VRE. Benzothiazole derivatives with a 1,4-disubstituted 1,2,3-triazole ring (88‒97, 108‒117 and 118–156) were also tested for antiviral activity. In order to establish a possible correlation between antitumor and antioxidative activity, derivatives that showed pronounced antiproliferative activity were evaluated for antioxidant activity in vitro by the DPPH method. Among all the prepared derivatives the most significant antiproliferative activity in vitro showed 6-chloro-2-(4-N,N-diethyl-3-fluorophenyl)benzimidazole (23) (K-562, Z-138, IC50 = 2.0 μM), 6-chloro-2-(4-N,N-diethyl-3-methoxyphenyl)benzothiazole (59) (CFPAC, IC50 = 1.03 μM) and (E)-6-chloro-2-(3-(4-(N,N-dimethylethoxy)-3-fluorophenyl)hidrazonyl)benzothiazole (188) (CAPAN, IC50 = 0.6 μM, NCI-H460, IC50 = 0.9 μM). The strongest selective antibacterial activity was shown by 2-(4-alkoxyphenyl)-6-chlorobenzimidazole derivatives (15‒17) on Enterococcus faecalis (MIC = 0.25–1 mg/mL) and 6-chloro-2-(4-N,N-diethylphenyl)benzothiazole (50), which showed the most pronounced activity against MRSA strain 13276 (MIC=2 μg/mL). Triazole derivative of 2-chlorobenzothiazole (89) showed the strongest antiviral activity against influenza virus H1N1 (EC50 = 6.7 μM).