Critically ill patients require special medical care due to severely impaired vital functions. The basis for assessing the patient's condition is a clinical examination, constant monitoring and re-evaluation of the condition. Important additional help is provided by diagnostic tests, including biomarkers. Biomarkers represent a broad group of clinical signs that can be accurately measured, and one of them is procalcitonin (PCT). Under physiological conditions, PCT is a precursor of calcitonin, a hormone crucial for calcium regulation. During a bacterial infection, the blood level of PCT rises due to the influence of microbiological endotoxins and inflammatory mediators. PCT levels begin to rise as early as 2-6 hours after the onset of the disease, reach their maximum value in the first 6 to 24 hours, and decrease rapidly with the resolution of the infection. In the group of acutely ill patients of all ages, PCT is most commonly used as a biomarker of bacterial infection. Numerous studies in the adult population have demonstrated good accuracy of PCT as a diagnostic and prognostic biomarker of sepsis. It also has a role in the implementation of antibiotic treatment, i.e. as an aid in deciding on the de-escalation of antimicrobial therapy. Additionally, PCT helps to distinguish inflammation caused by bacteria from other causes, including viruses. In the pediatric population, PCT is mainly used as an additional diagnostic biomarker of sepsis, including early and late onset neonatal sepsis, and less as part of antibiotic treatment strategies. A relatively common area of application is also urinary tract infections in children, where PCT is used as an indicator of upper urinary tract involvement. Although there is a lot of data on the usefulness of PCT in these areas of application, more research is needed, especially in the pediatric population. Also, it is important to emphasize that all biomarkers, including PCT, can be an aid in clinical decision-making, but that despite the progress in diagnostics, thorough clinical examination and constant monitoring of patients still are the basis of good medical care.