This is a revised and expanded version of the earlier edition. The new material is on Markov-modulated storage processes arising from queueing and data commu- nication models. The analysis of these models is based on the fluctuation theory of Markov-additive processes and their discrete time analogues, Markov random walks. The workload and queue length processes, omitted from the earlier edition, are also presented. In addition, many sections have been rewritten, with new re- sults and proofs, as well as further examples. The mathematical level and style of presentation, however, remain the...