Abstract: Triggered by strong disturbances, linearly stable channel flows may turn to be turbulent at moderate Reynolds numbers, and encouraging progresses have been made on the study of these subcritical transitions in the past two decades. For plane-Poiseuille flow, the initial stage of the transition is a sparse turbulent state, where large-scale oblique turbulent bands are composed of small-scale vortices and high and low speed streaks, separated by large laminar regions, and are able to extend obliquely. At this stage, the turbulence fraction has an upper bound, but is not a single-valued function of the Reynolds number. With the increase of the Reynolds number, the equilibrium localized turbulence state is reached, i.e. when the channel domain is large enough the flow is statistically steady and its turbulent fraction is a single-valued function of the Reynolds number and can be described by the Directed Percolation model. When the Reynolds number is increased further, band split occurs more frequently and the turbulent bands spread to the whole flow domain eventually, and the flow turns to be uniform turbulence at higher Reynolds numbers. The dynamic models proposed for the subcritical transitions are summarized, and the local stability parameters to quantitatively characterize the similarity among the transitions of pipe flow, plane-Couette flow, and plane-Poiseuille flow are surveyed. Finally, the development of the study on the subcritical transitions in channel flows is prospected.