This thesis explores the concept of neuronal communication through oscillatory synchronization. For most of the described research, we used the human motor system as a model system, in particular the cortico spinal system, in combination with non invasive recording techniques. Oscillatory synchronization is a well known property of neuronal activity in the motor system, both within brain regions, and between brain regions and the spinal cord. We used the coherence measure to quantify oscillatory synchronization between neuronal groups. We tested the underlying hypothesis that oscillatory synchronization subserves neuronal communication. We took three different approaches to investigate coherence: 1: Chapter 2 and 3 present data of experiments in which the relation between oscillatory neuronal synchronization and behaviour was investigated. Chapter 2 describes an experiment that provides support for the hypothesis that oscillatory synchronization within and between neuronal groups enhances neuronal communication and is behaviourally relevant. In chapter 3 an experiment is described, in which local and long range oscillatory synchrony was studied in the human motor system during a cued reaction time task. 2.: Chapter 4 describes a study in which coherence was used as a signature of neuronal interaction to identify a functional network involved in motor control. 3.: In chapter 5 and 6 we adopt a more mechanistic view on coherence. We test the prediction that phase locked oscillatory activity between neuronal groups makes their interaction effective and selective. The data presented in chapter 5 were obtained from the human motor system, and the data presented in chapter 6 were from the visual system of awake cats and monkeys