In a Wide Area Network (WAN) established infrastructure, one of the main problems ATM network planners and users face, when greater than T1/E1 bandwidth is required, is the high cost associated to T3/E3 links. The technology to cover the gap between T1/E1 and T3/E3 bandwidth at reasonable cost is known as Inverse Multiplexing for ATM (IMA). IMA allows multiple T1/E1 lines to be aggregated to support the transparent transmission of ATM cells over one single virtual trunk. In this paper, the fundamentals and major applications of IMA technology are described. In addition, the behavior of IMA multiplexers is carefully analyzed and a method to dimension them proposed. For that purpose an IMA simulation tool has been developed, which permits the study of individual devices and the evaluation of the end-to-end performance of a logical trunk under several ATM input traffic patterns. The analytical study is based on the comparison with a M/D/C/(N+C) queue under Poisson input traffic.