Knowledge graphs have emerged as a powerful paradigm for organizing and integrating structured knowledge in smart digital libraries (SDLs). This chapter provides an overview of knowledge graphs, their key concepts, applications, and the underlying techniques involved in their construction and utilization. The role of knowledge graphs in SDLs is explored, highlighting their capacity to enhance discoverability, accuracy, and personalization of library services by integrating diverse collections, metadata, and external resources. The chapter delves into implementing knowledge graphs in library settings, discussing data modeling, technology selection, and the importance of collaboration among stakeholders. Various real-world applications and case studies are presented, showcasing the benefits of knowledge graphs in enhancing resource discovery, data integration, and user experience. Challenges associated with knowledge graph implementation, such as data complexity, scalability, and maintenance, are addressed, along with potential solutions and best practices. The chapter also explores the interoperability and integration of knowledge graphs with existing library systems and emphasizes the importance of maintenance, quality assurance, and continuous enhancement of knowledge graphs. Looking towards the future, the chapter discusses emerging trends and directions, including the impact of semantic web technologies, artificial intelligence, and machine learning on developing intelligent and user-centric information environments. Potential applications, such as intelligent knowledge discovery, decision support systems, automation, personalization, and collaboration, are highlighted, along with associated challenges and considerations. This chapter comprehensively overviews SDL knowledge graphs, their current applications, challenges, and future potential. It emphasizes their transformative role in enhancing knowledge organization, discovery, and user experience in the digital library landscape.