Functional transcranial Doppler sonography (fTCD) constitutes a complementary neuroimaging tool measuring cerebral perfusion changes due to neural activation. Functional TCD utilizes pulse-wave Doppler technology to record blood flow velocities in the anterior, middle, and posterior cerebral arteries. Comparable to other perfusion-sensitive neuroimaging techniques like functional magnetic resonance imaging or positron emission tomography, fTCD is based on a close coupling between regional cerebral blood flow changes and neural activation. Due to a continuous registration of blood flow, TCD offers an excellent temporal resolution in comparison to other neuroimaging techniques. The technique is noninvasive and easy to apply. Blood flow measurements are robust against movement artifacts. Thus, fTCD is predestinated for follow-up investigations, especially in individuals with diminished ability to cooperate, like patients or children. Since its introduction the technique has contributed substantially to the elucidation of the hemispheric organization of cognitive, motor, and sensory functions in adults and children. This chapter delineates the physical and physiological principles of the technique. A prototypical experimental setup and analysis techniques are described. Scientific and clinical applications of fTCD are presented.