The cerebellum plays an essential role in movement execution and motor control by modulating the primary motor cortex (M1) through cerebellothalamocortical connections. Transcranial electrical and magnetic stimulation (TMS) allows direct investigations of neural networks by stimulating neural structures noninvasively in humans. The motor evoked potential (MEP) to single-pulse TMS of M1 is used to measure the motor cortical excitability. A conditioning stimulus over the cerebellum preceding a test stimulus of the contralateral M1 enables us to study the cerebellar regulatory effects on M1. In this chapter we describe this cerebellar stimulation method and its usefulness as a diagnostic tool in clinical neurophysiology.