A number of different forms of Orthogonal Frequency Division Multiplexing (OFDM) have recently been developed for application in a range of optical systems. In this presentation the key differences between these are explained. Analytical techniques developed for conventional optical modulation formats cannot always be applied successfully to optical OFDM. For example, in calculating the effect of nonlinearities in OFDM, the overall shrinkage or rotation of the constellation may be a significant effect. Depending on the properties of phase noise, the distortion caused may be easily corrected in the digital section of the OFDM receiver. copyright (c) 2008 IEICE.