This paper reviews the current state of understanding of the L-H transition phenomenon in tokamak plasmas with a focus on two central issues:(a) the mechanism for turbulence quick suppression at the L-H transition;(b) the mechanism for subsequent generation of sheared flow.We briefly review recent advances in the understanding of the fast suppression of edge turbulence across the L-H transition.We uncover a comprehensive physical picture of the L-H transition by piecing together a number of recent experimental observations and insights obtained from 1D and 2D simulation models.Different roles played by diamagnetic mean flow,neoclassical-driven mean flow,turbulence-driven mean flow,and turbulence-driven zonal flows are discussed and clarified.It is found that the L-H transition occurs spontaneously mediated by a shift in the radial wavenumber spectrum of edge turbulence,which provides a critical evidence for the theory of turbulence quench by the flow shear.Remaining questions and some key directions for future investigations are proposed. This paper reviews the current state of understanding of the LH transition phenomenon in tokamak plasmas with a focus on two central issues: (a) the mechanism for turbulence quick suppression at the LH transition; (b) the mechanism for subsequent generation of sheared flow. We briefly review recent advances in the understanding of the fast suppression of edge turbulence across the LH transition. We uncover a comprehensive physical picture of the LH transition by piecing together a number of recent experimental observations and insights obtained from 1D and 2D simulation models. Different roles played by diamagnetic mean flow, nebullass-driven mean flow, and turbulence-driven mean flow, and turbulence-driven zonal flows are discussed and clarified. It is found that the LH transition occurs spontaneously mediated by a shift in the radial wavenumber spectrum of edge turbulence, which provides a critical evidence for the theory of turbulence quench by the flow shear. Reliance questions and some key directions for future investigations are proposed.