Wireless Local Area Networking standard (Wi-Fi) and the WPAN standard (Bluetooth and Zigbee) products utilize the same unlicensed 2.4 GHz ISM band. Co-existence between such wireless technologies within the same frequency spectrum is crucial to ensure that each wireless technology maintains and provides its desired performance requirements. This paper provides a brief description of the newly introduced Zigbee standards including the Physical (PHY) and media access control (MAC) layer. It focuses on developing MatLab/Simulink models for the Zigbee protocol and the performance evaluation of these models. Several simulations were run and the results were analyzed for the different scenarios. The results showed how the relationship between the signal Bit Error Rate (BER) and Signal to Noise Ratio (SNR) was affected when varying the data rate and power. Furthermore, this paper investigated the co-existence of WLAN (IEEE 802.11g) with Zigbee (IEEE 802.15.4 by quantifying potential interferences and examining the impact on the throughput performance of IEEE 802.11g and Zigbee devices when co-existing within a particular environment. The effect of Zigbee on IEEE 802.11g was compared with the effect of Bluetooth under the same operating conditions.