The scope of this chapter is limited to the dynamics of lava and gas rising in a volcanic conduit. No mention will be made of the behaviour of eruption columns in the atmosphere, which would deserve a chapter on their own. Some basic knowledge of the main principles of fluid dynamics is assumed. The text contains no complex mathematical derivation and, save for a few simple points, details all the steps required to arrive at a given solution. The aim of this chapter is to highlight a few key physical principles using simple problems of relevance to volcanic eruptions. True eruptions are of course more complex than the examples given and involve various interactions between the variables.