We consider queueing systems in which the ser,ice rate is the controlled va
riable. The cost depends on the queue length and selected rate. The objecti
ve is to choose the service rate dynamically, based on the state of the sys
tem so as to minimize the average cost over an infinite horizon. Six classe
s, either known in the literature or new, are studied in detail: queueing s
ystems with vacations, single-server queueing systems with and without swit
ching costs, and tandem queueing systems with and without service costs. A
novel approach is presented here using fuzzy control to solve these problem
s. Simulation shows that the approach is efficient and promising, especiall
y in cases where analytical solutions do not exist.