The sample preparation techniques for biological TEM analysis have been improved significantly since the method of high pressure freezing (HPF) has been developed and optimized HPF machines are available. HPF technique employs cryo-immobilization of cellular compounds by solidifying cell water, and thus preserves the native state of a sample by the ultra-fast cryo-immobilization. This is different from the conventional preparation techniques using chemical fixation, where degradation and diffusion processes can cause preparation artifacts. However, a disadvantage of HPF method is that freezing can lead to heavy destruction of cells and intracellular structures due to ice crystal formation. Furthermore, to protect the TEM vacuum, the water has to be removed from the samples after the freezing process. This process is called freeze substitution and also takes place under low temperature.