This book is concerned with the study of non- Markovian queueing systems and networks, with applications to telecommunication systems. Its main contribution consists in deriving results for non- Markovian systems that have been obtained so far only for Markovian queueing systems. We study large closed client/server communication networks and losses in single-server queueing systems, with an application to communication networks of loss queues. The main results of this study are (i) an explicit expression for the interrelation between the limiting non-stationary distributions in non- bottleneck client stations; (ii) derivation of diffusion and fluid approximations for the non- Markovian queue length in the bottleneck client station. For the loss networks considered, we find an asymptotic expression for the loss probability and other performance measures, as buffer capacity increases to infinity. We also find the changes in the loss probability when redundant packets are added to the messages.