Transcranial magnetic stimulation is increasingly used as a tool to explore cortical motor function in healthy subjects and in patients with neurological disease or injury. This review describes a "twin coil" TMS approach that allows investigation of time related changes in functional connectivity between primary motor cortex and other areas in preparation for a forthcoming movement. Investigations into premotor-motor interactions show that these are specific to the type of task that is performed as well as the muscles used to control the movement, allowing us to monitor information flow within motor networks with millisecond time resolution.