This research work consists of synthesis of various thiosemicarbazides and 1,2,4-triazole derivatives and screening of their biological activities. All compounds were fully characterized by various spectroscopic techniques, such as 1H-NMR, 13C-NMR, and EIMS/FABMS. Melting points of all compounds were also recorded. This dissertation consists of two chapters based on the extensive literature and research findings regarding the four libraries of synthetic compounds. Each chapter has its own compounds numbering, tables, figures, schemes, and references. Chapter-1 deals with general introduction of thiosemicarbazides, their previous synthetic strategies, and their biological activites. It also describes general introduction of biological activities and their bioassays. It is comprised of the synthesis of various derivatives of 4chlorophenyl substituted thiosemicarbazides 33-57 (Part A), and nicotinic/isonicotinic substituted thiosemicarbazides 60-84 (Part B) and their in vitro activities against urease, α-glucosidase, and acetylcholinesterase (AChE) enzymes. Compounds 35, 42, 46, 49, 61, 67, 77, and 79 were new derivatives while rest of the compounds were previously known. Except few all synthetic compounds showed superior activity than the standard thiourea. Compound 57 was sixty six fold, compound 42 was nineteen fold, compounds 35, 38, 52 were about ten fold and compounds 69 and 81 were eighteen fold more potent than the standard thiourea. Some synthetic thiosemicarbazides showed weak activity against αglucosidase enzyme while showed no activity against acetylcholinesterase enzyme. Chapter-2 deals with general introduction to 1,2,4-triazole, their previous synthetic strategies, and their biological activites. It is also composed of the synthesis of various analogues of 4-chlorophenyl substituted 1,2,4-triazole derivatives 156-180 (Part A), and synthesis of thioether derivatives of 1,2,4-triazoles 181-192 (Part B) by four steps reaction and their in vitro activities against urease, α-glucosidase, and AChE enzymes. Compounds 158, 162, 163, 165, 167, and 169-173 were new derivatives, while rest of the compounds were previously reported by others. 1,2,4-Triazole derivatives 156-180 showed good to excellent urease inhibitory activities. Compounds 156, 163, 166, and 176 were more potent compounds, particularly, compound 176 showed 28-fold more potent activity than the standard thiourea. Compounds 156, 162, 163, 166, 175, and 179 exhibited weak αglucosidase inhibitory activity, while 1,2,4-triazole derivatives 156-180 showed no activity against AChE enzyme. Thioether derivatives 181-192 showed a weak inhibitory activity against urease enzyme, while good to weak inhibitory activity against α glucosidase enzyme, particularly, thioether derivatives 182 and 185 were found to be the more potent than the standard acarbose. All 1,2,4-triazole derivatives 156-180 showed no activity, while thioether derivatives 181-192 showed weak inhibitory activity against AChE enzyme.