Functional Disorders are common medical problems both in primary and in secondary health care. The mechanisms that cause symptoms such as primary pain, fatigue, dizziness are still unknown. Various classifications, including ICD-10 or DSM-5, describe these conditions differently, and new proposals are being developed e.g. in ICD-11, RDoC. Many controversies are evoked by lack of unequivocal explanatory theory. The early psychoanalytical concept has been modified by other explanations, such as immunological abnormalities, dysfunction of vegetative system and HPA axis, central sensitization, diverted processes of perception or predictive processes within cognitive homeostasis dysregulation. Insufficient scientific evidence makes therapies unsuccessful and justifies further study. Psychotherapy, pharmacology and complementary medicine are supplemented by new experimental methods of treatment connected with progress in neuroscience. The recently developed non-invasive Transcranial Direct Current Stimulation (tDCS), Transcranial Magnetic Stimulation (TMS) and – neurofeedback (EEG-NF), based on EEG registration, are undergoing tests. Applying complex mathematical algorithms to localized bioelectrical signal sources makes it possible to modulate and reshape connections of neuronal networks within specific cortex areas. This article presents the current state of knowledge concerning functional disorders, highlighting the ways in which different definitions of FD have an impact on approaches to treatment.