Abstract: Notions from thermodynamic formalism such as pressure, equilibrium states and large deviations can give a rich qualitative description of a dynamical system. Recently there has been a lot of activity in the development of thermodynamic formalism applied to non-uniformly hyperbolic dynamical systems. These systems have been shown to exhibit a wide variety of phenomena, most interestingly critical phenomena such as phase transitions. In this talk I will give a fairly complete description of the possible behaviour of the class of unimodal interval maps, including the relation between phase transitions and the existence of a natural measure for the system.