The potential future development of quantum computers, which could threaten the security of current networks, underscores the importance of planning for the transition to quantum-safe solutions. One approach under consideration, and the focus of this chapter, is the development of quantum key distribution (QKD) networks, which enable the secure distribution of secret keys between pairs of users within the network. At the foundation of a QKD network is a QKD link, which consists of a single optical connection between two QKD devices. This chapter explores the technology behind QKD links, introduces various models of QKD networks, and highlights real-world implementations being pursued by several countries worldwide.