The M/G/1 theory is a powerful tool, generalizing the solution of Markovian queues to the case of general service time distributions. There are many applications of the M/G/1 theory in the field of telecommunications; for instance, it can be used to study the queuing of fixed-size packets to be transmitted on a given link (i.e., M/D/1 case). Moreover, this theory yields results which are compatible with the M/M/1 theory, based on birth-death Markov chains.In the M/G/1 theory, the arrival process is Poisson with mean arrival rate λ, but, in general, the service time is not exponentially distributed. Hence, the service process has a certain memory: if there is a request in service at a given instant, its residual service time has a distribution depending on the time elapsed since the beginning of its service. Let us refer to a generic instant t. The system is described by a two-dimensional state S(t), characterized as follows:• Number of requests in the system at instant t, n(t).• Elapsed time from the beginning of the service of the currently served request, τ(t). Note that in the Markovian M/M/1 case, the pdf of the residual service time does not depend on τ(t) because of the memoryless property of the exponential distribution.Hence, S(t) ¼ {n(t), τ(t)}. In order to characterize these queues, we study their behaviors at specific time instants ζ i where we obtain a mono-dimensional simplification of state S(ζ i ). The M/G/1 queue is studied at specific imbedding instants, where we obtain again a Markovian system; this is a so-called imbedded Markov chain [1,2]. Different alternatives are available to select instants ζ i . It is not requested that instants ζ i be equally spaced in time. Typical choices for ζ i instants are:The online version of this chapter (