In the last few years, there is a rapid growth in the area of wireless communications and the demand of limited wireless spectrum has increased significantly. In early days, the spectrum was assigned by following static allocation schemes. This fixed spectrum assignment policy results in under-utilization of scarce radio spectrum.The study by Federal Communications Commission (FCC) shows that the spectrum is available but its usage is not efficient. Cognitive Radio is a new technology that increases the utilization of available spectrum by implementing dynamic allocation schemes.In this book, an Opportunistic Cognitive MAC Protocol (OC-MAC) has been proposed for the Cognitive Radio Networks to access the unoccupied spectrum opportunistically and coexist with Ad hoc Wireless Local Area Network (WLAN).By implementing OC-MAC protocol,the utilization of spectrum increases significantly while collision rate among different types of users remains under acceptable level. All simulation scenarios are developed in OPNET v14.5 Network simulator and simulation results are collected for multiple scenarios; under light and heavy traffic to check the performance of the OC-MAC protocol.