ThisFluidssingle-component chapter addresses single-component fluids, i.e. pure fluids that do not mix or react with other chemical components. In Part I, idealised single-component fluids have already been presented: Ideal gases, which follow the known thermal and caloric equations of state, and incompressible liquids and solids, respectively, whose specific volume remains constant. For incompressible liquids/solids, the caloric equations of state can be easily modified. However, the behaviour of real fluids differs from the idealised approaches. In particular, real fluids can change their state of aggregation. Phase changes occur in many ways in nature and technical applications, e.g. melting or vaporisation of water, in humid air or in combustion processes. Therefore, this first chapter of Part II is essential for further understanding. The first goal is to describe the thermodynamic state of real fluids.