Nitrogen oxides (NOx) are highly reactive air pollutants that cause various environmental and health concerns. Selective catalytic reduction (SCR) has emerged as a vital tool for mitigating NOx emissions. This comprehensive review explores the advancements in SCR technologies and their critical role in NOx reduction by investigating various reducing agents. The performance of ammonia/urea, hydrocarbons, hydrogen, and carbon monoxide as reductants was investigated in various SCR systems. Ammonia, which is renowned for its efficiency in SCR, was examined for its ability to reduce NOx emissions. Urea-based SCR is a safe and low-toxicity alternative, particularly for mobile applications. Hydrocarbon-based SCR systems are versatile, enabling lower operating temperatures and improved efficiency. Hydrogen-based SCR technology is an environment-friendly option for NOx reduction, particularly in low-temperature scenarios. Carbon monoxide-SCR optimizes reduction processes, particularly in industrial sectors like steel and coking where CO abundance is beneficial for NO removal, addressing critical emission reduction needs. In-depth information on these reducing agents was examined, along with information on the catalyst components, mechanisms, and important factors that affect the NOx conversion efficiency. This study also highlights the need for research to address several challenges, future directions, and innovations in SCR technologies, emphasizing the importance of reducing environmental impacts and enhancing industrial processes.