Abstract. This paper reviews processes which, it is argued, occurred in lunar lava flows and were involved in the formation of lunar sinuous rifles.The development of ideas relating to rille formation is described and evidence which suggests that they are products of lava effusions is outlined.An analytical lava flow model is described which relates the shapes of lava flows to conditions at the commencement of the flows. An essential part of the model is its use of the yield strength of lava as a parameter. It is shown that the model predicts the occurrence of the channel/levee morphology of lava flows.It is argued that the channels of large flows were deepened by melting. Heat transfer processes within lava flows are discussed and analysed and it is shown that a prolonged period of flow is required before melting of the substrate begins. Estimates are given of initial heating periods and subsequent melting rates.The possibility of the occurrence of turbulent lava flow is discussed. Application of the flow model to sinuous rilles and tlows in Mare Imbrium shows that both types of feature could be the remnants of similar effusive events in which the flow could have become turbulent. Turbulent flow is shown to promote erosive melting and it is suggested that the sinuous rilles were cut by the flows which became turbulent.A process of rille formation is described which is consistent with the foregoing arguments and with observations made at Hadley Rille. It is concluded that rilles were formed by the rapid effusion of large volumes of low yield strength lavas.