Over the past 20 years, neuroimaging has become a predominant technique in systems neuroscience. One might envisage that over the next 20 years the neuroimaging of distributed processing and connectivity will play a major role in disclosing the brain’s functional architecture and operational principles. The inception of this journal has been foreshadowed by an ever-increasing number of publications on functional connectivity, causal modeling, connectomics, and multivariate analyses of distributed patterns of brain responses. I accepted the invitation to write this review with great pleasure and hope to celebrate and critique the achievements to date, while addressing the challenges ahead. Key words: causal modeling; brain connectivity; effective connectivity; functional connectivity

We give two new approaches to testing a conditional form of exogeneity. This condition ensures unconfoundedness and identi cation of e ects of interest in structural systems. As these approaches do not rely on the absence of causal e ects of treatment under the null, they complement earlier methods of Rosenbaum (1987) and Heckman and Hotz (1989).