This chapter describes various mechanisms of radio propagation. It discusses the effect of interference on Wireless local area network (WLAN) performance. The chapter analyses the effect of multipath propagation on WLAN performance. It shows that the effect of signal attenuation on system performance. The chapter also describes received signal strength, signal-to-noise ratio, signal-to-interference ratio, and bit error rate. There are three basic mechanisms of radio propagation in wireless and mobile communications. These are reflection, diffraction, and scattering. A wireless channel is affected by the distance between a transmitter and a receiver, propagation characteristics of radio signals in certain environments, obstacles due to office walls and corners, and the presence of other devices operating in the same frequencies. The radio signal would reflect from objects during transmission. The state of radio signal plays an important role in determining the overall performance of WLANs.