Multielectrode neurophysiological recording and functional brain imaging produce massive quantities of data. Multivariate time series analysis provides the basic framework for analyzing the patterns of neural interactions in these data. Neural interactions are directional. Being able to assess the directionality of neuronal interactions is thus a highly de-sired capability for understanding the cooperative nature of neural computation. Research over the last 15 years has identified Granger causality as a promising technique to furnish this capability. In this talk, I will introduce the concept of Granger causality and present results from applications of this technique to electrophysiological and function-al imaging data.