Nuclear security capacity building is the systematic process of developing and strengthening the technical, human, and institutional capabilities required by states and relevant organizations to prevent, detect, and respond to malicious acts involving nuclear and other radioactive material. As a research field, it investigates the strategies, mechanisms, actors, challenges, and effectiveness of these capacity enhancement efforts across diverse contexts, including policy frameworks, regulatory structures, physical protection systems, personnel training, and international cooperation initiatives. Its key characteristics include its multidisciplinary nature and focus on sustainability, with its significance residing in its vital contribution to mitigating global nuclear security risks and preventing nuclear terrorism.