This synthetic review of gully morphology and genesis focuses on incised semi-permanent gully systems rather than on shallow ephemeral gullies. It examines the conditions for gully formation; a sharp step to initiate a headcut, a sufficiently low effective bedload fraction to evacuate eroded material, and the potential to maintain steep sidewalls, usually dominated by mass movement processes. Gully formation is also favoured by an indurated surface layer which maintains steep sideslopes, often with armouring material from the capping layer, and a sharp headcut which does not diffuse away. Two different approaches towards the areal modelling of gully system development agree in treating the ratio of advective (channel) to diffusive (sideslope) processes as a key determinant of the morphology of a gully system as it evolves. Implications for gully prevention and remediation are briefly discussed.