Mechanisms for the flow of liquid over outlet weirs of distillation and absorption trays are described in this paper. Weir flow contributes to the liquid height on a tray, and this height is important for the design and operation of trayed columns. Comparisons of available experimental data with Francis＇ weir equation and with new stochastic liquid flow models showed that the traditionally used Francis＇ equation needs modification. The stochastic flow models Link the weir flow to the flow mechanisms upstream of the tray outlet. These upstream mechanisms provide the input needed for a new weir crest equation. The existence of a so-called ＇edge-vortex＇ (a horizontal roll cell at the boundary of the gassed and the ungassed region) is shown to play an important role, when a calming zone is present upstream of the outlet weir.