I propose the theoretical study of multiple-access quantum key distribution (QKD) networks. Quantum cryptography is an emerging interdisciplinary field of science and engineering with developing commercial applications. It is one of the objectives of the work programme to provide European citizens with the security level that the quantum technology offers. Multiple-access QKD networks make us one step closer to this goal, and open up new marketing opportunities by introducing more flexible configurations as compared to the current point-to-point QKD links. In a multiple-access network, every two users can use a shared channel to exchange a secure key without trusting any other users. This shared channel can be used, possibly, at the same time, by other pairs of users who also wish to create secure keys. Such an advantage becomes possible by employing the well developed multiple-access techniques in classical optical communication networks. In particular, I intend to study the three schemes of wavelength, code, and time division multiple access, in which, respectively, each user is assigned a particular wavelength, code, or time slot. The main question to answer is how the interference caused by other users will affect the rate of secure key generation in a multiple-access QKD setup. It is also important to compare different network configurations in terms of their architecture and cost. Finally, I would like to investigate the possibility of extending the ideas relevant to local and metropolitan area networks, to wide area networks, where we need to employ quantum repeaters for the secure exchange of the key. Experimental demonstration of such networks will also be pursued via finding other sources of funding or in collaboration with other research groups.