In order to satisfy the service demands with different granularities and maximize the spectral efficiency of wavelength division multiplexing transmission,the optical network is evolving to agile and energy-efficient one.Consequently,the flexible optical transmission techniques based on bandwidth variable transceivers (BVT) have attracted worldwide attention and research interest.We review recent progresses of flexible optical transmission techniques including transmitter-side adaptive modulation,variable bit-rate loading and receiver-side corresponding modulation formats identification.Finally,we outline the prospects and future challenges in this area.