Functional magnetic resonance imaging (fMRI), with its unique advantages of non-invasiveness, relatively balanced spatial and temporal resolution, repeatability, and whole brain imaging, has brought methodological breakthroughs to neuroscience and neurological clinical practice. By measuring blood oxygen-dependent signals, fMRI can reveal brain activity in task execution, sensory stimulation, and emotional regulation. By comparing the brain functional activity between patients and normal controls, fMRI can detect the abnormal brain regions associated with mental illness. During the rehabilitation of stroke, traumatic brain injury, and neurodegenerative diseases, fMRI enables the evaluation of brain function recovery and guides personalized training. This chapter introduces the working principle of fMRI and summarizes the state of the art, providing references for clinicians, researchers in neuroscience, and biomedical engineers.