At low temperatures it is no longer possible to consider the motion of individual atoms. and a wave mechanical treatment becomes necessary . Such a treatment. as developed by Landau and Feynman. characterizes the thermal motion of the liquid by ' elementary excitations .. the so-called phonons and rotons . A consideration of these excitations then leads to the ' twofluid' model . It is shown that the treatment is adequate to account for nearly all the properties of the liquid. which are briefly reviewed . The principal problem still outstanding concerns the behaviour of the liquid at high rates of flow or when carrying a large heat current. when it is clear that some form of turbulence is induced .An account is given of the present state of the theory of liquid 4He .