The focus of this thesis is on the synthesis and in vitro biological testing of some target N4- substituted isatin-3-thiosemicarbazones and their transition metal complexes. Thus, three series of N4-benzyl substituted isatin-3-thiosemicarbazones (19-33), (34- 48) and (49-63) were synthesized by condensing isatin, 5-nitroisatin and 5-chloroisatin, respectively, with the appropriate N-substituted thiosemicarbazides. All the synthesized thiosemicarbazones (19-63) were characterized by means of their analytical (CHN) and spectral (IR, 1H-NMR, 13C-NMR, EIMS) data and tested for the selected biological properties i.e. antiurease, phytotoxic, cytotoxic, anticarbonic anhydrase and antiglycation activities. All the synthetic thiosemicarbazones (19-63) proved to be highly potent inhibitors of urease, displaying excellent inhibitory activity (IC50 = 0.87 ± 0.25 − 11.23 ± 0.19 µM) even better than the reference inhibitor, thiourea (IC50 = 22.3 ± 1.06 µM) used in the assay. In phytotoxicity assay, 33 out of 45 thiosemicarbazones tested i.e. (19-22), (25), (26), (28-30), (32), (33), (34-40), (42-44), (46-48), (51), (52), (56-61) and (63) appeared to be active, exhibiting weak or non-significant (5-100%) growth inhibition of Lemna aequinocitalis at 1000 or 500 µg/mL concentrations in comparison to paraquat (the standard herbicide), which showed 100% plant growth inhibition at 0.015 µg/mL concentration. In the brine shrimp (Artemia salina) lethality bioassay, only 4 compounds i.e. (20), (28), (33) and (42) were found to be active, demonstrating cytotoxic activity with LD50 values 3.63 × 10-5, 2.90 × 10-5, 2.31 × 10-4 and 2.55 × 10-5 M, respectively. The remaining compounds gave LD50 values greater than 2.36 × 10-4 − 3.22 × 10-4 M and were, therefore, considered to be almost inactive. On the other hand, in the carbonic anhydrase (CA-II) inhibition bioassay, all the trial compounds (19-63) showed less than 50% of enzymatic inhibition and thus were considered to be inactive. However, in the antiglycation activity assay, 21 out of 45 compounds tested i.e. (20-22), (24), (26), (28), (29), (34), (36), (39), (45), (47), (48), (50), (51), (56-59), (61) and (62) proved to be potent inhibitors of glycation, demonstrating inhibition with IC50 values ranging from 114.51 ± 1.08 to 643.80 ± 5.80 µM. Of these, (21), (22), (26), (28), (29), (34), (36), (51), (56-58) and (62) exhibited superb inhibitory activity (IC50 = 114.51 ± 1.08 – 241.90 ± 1.97 µM) even better than the reference inhibitor, rutin (294.50 ± 1.50 µM) and thus may act as convincing leads for further studies. The synthetic thiosemicarbazones (19), (21), (22), (24-31), (34-37), (39-42), (44-46) and (48) were used as ligands for synthesizing their Cu(II) complexes. All the synthesized metal complexes (64-86) were characterized by means of their analytical (CHN), spectral (IR, R (Raman), UV-Vis), magnetic moments, thermal and molar conductance data, and evaluated for the selected biological properties viz. antiurease, anticarbonic anhydrase and antiglycation activities. In antiurease assay, coordination of all the thiosemicarbazone ligands to metal ions was found to lead to decrement in the enzyme inhibitory activity that they possessed. Relatively, extensive decrease occurred in the cases of (65), (67), (68), (71), (73), (76-81) and (84). On the contrary, in CA inhibition bioassay, coordination of the ligands to metal ions was found to give rise to induction of enzyme inhibitory activity in certain cases. For example, the metal complexes (66), (72-76), (80) and (83-86) exhibited promising enzymatic inhibition with IC50 values 5.9 ± 0.00 − 21.26 ± 0.35 µM in contrast to the corresponding ligands (22), (29-31), (34), (35), (40), (44-46) and (48), which displayed less than 50% (i.e. from − 9.3 to 33.1%) inhibition of the enzyme and thus considered to be inactive. Similarly, the coordination of thiosemicarbazone ligands to metal ions was found to bring about either induction or enhancement of antiglycation activity. For example, the metal complexes (64), (68), (70), (73), (74), (76), (78), (80-83) and (85) showed excellent activity with IC50 values ranging from 105.74 ± 3.1 to 247.06 ± 1.75 µM as compared to the relevant ligands (19), (25), (27), (30), (31), (35), (37), (40-43) and (46), which demonstrated less than 50% (i.e. 10.32−40.98 %) inhibition of glycation. Similarly, the metal complexes (65-67), (69), (71), (72), (75), (77), (79) and (84) displayed markedly enhanced antiglycation activity in comparison to the respective ligands (21), (22), (24), (26), (28), (29), (34), (36), (39) and (45) (IC50 values 94.64 ± 0.99 − 135.20 ± 1.87 vs. 209.87 ± 0.37 − 522.68 ± 9.1 µM).