The aim of this concise introduction to urodynamics is to describe the extent to which clinical observations can be accounted for by simple mechanical concepts, in order to enable medical physicists and bioengineers to interpret their observations of complex pressures and flows in the bladder and urethra. The peculiarities of flow in highly distensible tubes are described, and an explanation of the mechanical behaviour of smooth muscle is given. A prior medical or physiological knowledge is not assumed.