In this paper basic principles of functional magnetic resonance imaging are described. It is a radiologic technique that serves to visualize tissue functions based on observed changes in the magnetic field signal caused by blood flow, oxygenation and brain activity. The MRI signal is susceptible to changes in blood oxygenation so the physiologic base for fMRI is BOLD effect (blood – oxygen level dependent). In the blood cells there is hemoglobin which can be deoxygenated or oxygenated. Deoxygenated hemoglobin causes MRI signal changes by reducing it which represents the BOLD signal. Because of an interesting physiological phenomena called neurovascular coupling, these findings became a powerful tool for mapping brain activity. Increased neural activity leads to increased blood flow, but oxygen metabolism does not increase due to which we find low concentrations of deoxygenated hemoglobin in the blood resulting in stronger MRI signal. Some examples are described in this paper, but the connection between blood oxygenation and brain activity is still not completely clear. The brain activity lasts as long as the stimulus is on (task, sounds, words, visuals ...), but It takes time for the fMRI signal to rise and fall because of hemodynamic response so we can say that the fMRI signal is slow and delayed in time. These specified features of fMRI signals are resolution limitations at the same time. The time resolution of fMRI is 4 – 5 seconds, and the spatial resolution is about 3mm. Good resolution is necessary because of the large amount of data collected during the acquisition and potential artifacts and noise can affect the quality. The most common source of noise is the hardware of the device and the patient itself. The best way to remove nosie is by placing the patient in a comfortable position and using proper immobilisation. fMRI has found applications in both clinical and more basic neuroscience, especially for children. Localization of brain function, monitoring therapeutic and intervention effects, and diagnosis of mental illness are just some examples of what fMR can be used for.