The problem of reactive arthritis (ReA) is one of the most challenging in both adult and paediatric rheumatology, as many issues remain unclear.The initial concept of ReA as "sterile arthritis", in which the focus of infection is outside the joint, primarily in the urogenital tract or intestines, has been refuted to date. It has been proven that in ReA, individual microbial antigens and even microorganisms (chlamydia) can be isolated from joint tissues and synovial fluid.Epidemiological studies in ReA are limited due to the lack of unified diagnostic criteria, difficulties in examining patients, and the often asymptomatic course of the trigger infection. However, new pathogens have been identified, such as Mycoplasma pneumoniae, Giardia lamblia, Helicobacter pylori, Pseudomonas aeruginosa, Staphylococcus aureus, Staphylococcus haemolyticus, SARS-CoV-2 and others, which have become the etiological agent of overt ReA. The question of whether streptococcal arthritis, as well as arthritis after acute respiratory viral infections and post-vaccination arthritis, belong to ReA is still controversial.The pathogenesis and mechanisms of ReA development remain unclear. Although classical ReA is associated with HLA-B27, the role of HLA-B27 in ReA development is not fully understood. In most children, ReA is not associated with HLA-B27.Currently, there is no single classification of ReA and no generally accepted diagnostic criteria for the disease. The diagnostic criteria used are developed and focused only on adult patients. Very often, in paediatrics, the main criteria for the diagnosis of ReA are clinical and anamnesis data. In half of the cases, it is practically impossible to establish the etiological factor, and therefore, ultrasound examination of the joints plays a crucial role in the diagnosis of ReA in children.Recommendations for the treatment of ReA in both adults and children need to be updated. There are discrepancies in the recommendations for the treatment of ReA in adults and children. Medications used in adults are not always appropriate for children due to the age-related characteristics of the child's body and possible side effects.