The mid-latitude trough is a major feature of the F-region ionosphere that forms at the boundary between the mid-latitude and auroral ionospheres. The trough region has a major influence on the propagation of radio waves because of the large gradients in electron concentration, and the irregularities often embedded in the equatorward and poleward walls of the trough. The formation processes of the trough have been reasonably well understood for some time, and physically based models of the trough can now reproduce the climatology of the trough quite well. Prediction of the trough shape and dynamics for individual events, “trough weather,” is still under development. Recent scientific progress on major topics affecting trough morphology is described here, together with some suggestions on how the remaining uncertainties can be addressed.