In this book, a study of WIMAX performance using OPNET 14.5 IPv4 model for fixed and mobile WIMAX was considered. Several features were studied such as Automatic Repeat on reQuest (ARQ) for controlling errors techniques, Quality of Service (QoS) for different applications, pathloss propagation model and WIMAX handover. The simulation results of using ARQ shows high retransmission efficiency, the goal is to guarantee the reliable delivery of data packets. QoS parameters for Voice over Internet Protocol (VoIP) and File Transfer Protocol (FTP) applications were studied. Five different service classes were examined and discussed to study the effect of each class to those applications (Unsolicited Grant Services (UGS), real time Poll Services (rtPS), non real time Poll Services (nrtPS), extended real time Poll Services (ertPS) and Best Effort (BE)). Pathloss parameters were studied in various environments such as free space, outdoor to indoor and pedestrian, urban and suburban to show the effect of the distance between the transmitter and the receiver in different operating frequencies on the BS coverage area.