Orthogonal frequency divisionmultiplexing (OFDM)modulation has excellent performances, for example, strong tolerance against multipath interferences, effective spectral efficiency, high information capacity and simplicity of equalization. Consequently, it has been widely utilized in the services of digital terrestrial broadcasting, asymmetric digital subscriber line (ADSL), local wireless LAN and optical fiber communications. In the transmitter, relay station and receiver, signal processing techniques are used to mitigate the effects caused by various interferences, carrier frequency offset and noise, then to improve the equalization precision of information data. These techniques may achieve the utmost of their effectiveness if the reliable knowledge of the communication channel is applicable. Nevertheless, the prior information of the OFDM channel dynamics is typically unavailable, whereas the practical channel is often time-varying due to the differing propagation paths, scattering and reflection of electric waves. Hence it is necessary to identify the channel model from the observation data and some distinctive structural information inserted in the OFDM signals. In this chapter some channel identification problems as well as the fundamental mathematical tools are discussed, and several frequency domain algorithms are investigated.