  This is the transcript of a talk given at the 1992 Complex Systems Summer
School. The theory of large fluctuations of stochastically perturbed
continuous-time dynamical systems is reviewed, and the large fluctuations of
two stochastic models arising in computer science are analysed. One is a
stochastic model of a communications network, resembling an Ethernet, which
experiences traffic fluctuations. The other is a simple model of two dynamic
data structures in the same memory space. In both models `large fluctuations'
are unlikely, undesirable events. The mean time between occurrences of such
events is estimated.
