Current computer systems and communication networks tend to be highly complex, and they typically hide their internal structure from their users. Thus, for selected aspects of capacity planning, overload control and related applications, it is useful to have a method allowing one to find good and relatively simple approximations for the observed system behavior. This paper investigates one such approach where we attempt to represent the latter by adequately selecting the parameters of a set of queueing models. We identify a limited number of queueing models that we use as “Building Blocks” (BBs) in our procedure. The selected BBs allow us to accurately approximate the measured behavior of a range of different systems. We propose an approach for selecting and combining suitable BB, as well as for their calibration. Finally, we validate our methodology and discuss the potential and the limitations of the proposed approach.