<p>Quantum key distribution (QKD) is a cryptographic technology that supports the negotiation and sharing of private keys with unconditional security between authorized parties. As QKD scales to a global level, it must address performance limitations, high costs and practical security concerns. In this Review, we outline the key technical challenges, applications and prospective developments towards a global QKD network. Advances such as satellite-based QKD and newly developed protocols offer promising solutions for extending QKD over long distances. Field trials have progressively expanded from intercity links to larger-scale networks. Nevertheless, balancing cost–performance and security considerations will continue to challenge advanced research efforts. On the basis of the strategies addressing these obstacles, we highlight future directions that can support the efficient realization of global QKD infrastructures.</p>