This article summarises the main criteria of selecting working fluids for a thermodynamic cycle, such as heat engines including low grade heat recovery using Organic Rankine Cycle (ORC) for geothermal energy, waste heat, thermal solar energy or biomass and heat pumps and refrigeration cycles. The article addresses how working fluids affect technological applications, where the working fluid undergoes a phase transition and does not remain in its original (mainly gaseous) phase during all the processes of the thermodynamic cycle.