The transition towards Industry 5.0 emphasizes human-centricity, highlighting an irreplaceable role that humans play in smart manufacturing systems. Human digital twin (HDT) emerges as a pivotal enabler for human-centric smart manufacturing, facilitating the integration of human factors into cyber-physical systems. This chapter provides a comprehensive overview of HDT, detailing its definition, characteristics, and system framework. HDT represents a digital reflection of human multi-dimensional information, enabling two-way interaction between the physical and cyber worlds. It utilizes multi-source data to thoroughly describe individual characteristics and interactions with machines and environment. Enabling technologies also are explored, which enhance the capabilities of HDT in real-time monitoring, assessment, prediction, and interaction. Furthermore, various application of HDT in industrial scenarios are discussed, addressing technical and ethical challenges to ensure a safe and effective implementation of HDT in human-centric smart manufacturing.