Procalcitonin, a short form of glikoprotein is produced in the C-cells of the thyroid gland, which is changed to calcitonin by a specific protease. The blood level of procalcitonin in healthy people is under 0.1 ng/mL. During a severe bacterial, fungal or parasitic infection, the level of procalcitonin rises over 100 ng/mL and is produced in the tissues other than just the thyroid gland. During severe viral infections and noninfectious inflammatory reactions the level of procalcitonin does not rise, or rises very little. The level of procalcitonin rises 2000 times more in patients with septic shock. The levels of procalcitonin also rises after trauma, major surgical approaches and some malignancies. Cytokines play a very important role in severe infections such as sepsis and these may induce the production of procalcitonin. Procalcitonin, cytokines such as tumor necrosis factor (TNF), IL-1 and IL-6 are frequently elevated in patients with sepsis. The procalcitonin levels can increase for 24 hours. In differential diagnosis of severe bacterial infections, procalcitonin may be used as a valuable parameter. In this study, the value of procalcitonin in infectious diseases is discussed by comparing the literature.