Asynchronous transfer mode (ATM) is considered to be the ground on which B-ISDN is to be built. Understanding an ATM network before it is built is a challenging task as one must consider the design of the switch architecture, call processing, and signal processing subsystems at the same time. These components, already posing challenge for individual analysis, interact in complex ways. Libra is an object-oriented simulator developed for simulation and fast prototyping of ATM networks. It is designed and implemented to address the diverse issues encountered in the simulation of any large-scale ATM network. We discuss the unique features of Libra that makes it feasible for simulation of ATM networks and present the design and implementation issues of such a simulator. An example is also given to illustrate the application of Libra.