The brain contains nearly 100 billion neurons with an average 7000 synaptic connections. Recovering the exact wiring of the brain (connectome) at this neural level is therefore a daunting task. Traditional neuroanatomical methods of axonal tracing or electrophysiological direct assays cannot scale up to very large networks. Could there be alternative methods, such as inference algorithms, to recovering neural network structures from patterns of neural activity? This challenge (inaugurating a series of future "Connectomics" challenges) will stimulate research on network structure inference from calcium fluorescence data, including causal discovery methods. WHAT YOU GET: Time series of the activity of 1000 neurons in a culture (simulated calcium fluorescence data) WHAT YOU PREDICT: The directed connections between neurons. 45 days to go, 3000 $ allocated for the top three ranked competitors (if they accept to make an open release of their code ;-) Challenge submission website: https://www.kaggle.com/c/connectomics Over 70 teams are already attending! There will be $3000 in prizes awarded. We are an official challenge at IEEE WCCI 2014 meeting in Beijing, where challenge results will be spotlighted: http://www.ieee-wcci2014.org (see "accepted competitions") Participants may present their results at a dedicated workshop at ECML 2014 (Nancy, France): http://www.ecmlpkdd2014.org (see "discovery challenges") http://connectomics.chalearn.org/workshop (workshop webpage) The top-ranked challenge participants and authors of the best workshop papers will be invited to participate to full length submissions (individual or collective, including detailed post-challenge analytics and algorithm comparisons and benchmarking) to peer-reviewed journals, such as Frontiers in Neuroinformatics or PLoS Computational Biology. Calendar: May 5, 2014: End of challenge. May 20, 2014: All teams must turn in fact sheets. The three top ranking teams must turn in their code to qualify for prizes. The post-challenge verifications start. June 1, 2014: End of the post challenge verifications. Release of the official ranking. June 20, 2014: ECML workshop proceeding paper submission deadline. July 6-11, 2014: Result presentation at WCCI 2014, Beijing, China. August 30, 2014: Revised papers due for ECML workshop proceedings. Beginning of post-challenge papers design. September 15 or 19, 2014: ECML workshop, Nancy, France. Discussion of the results and preparation of post-challenge synthesis. For any information or request, please contact: Demian Battaglia, demian.battaglia@univ-amu.fr --- Demian Battaglia, PhD Institut de Neurosciences des Systèmes, Aix-Marseille Université - Marseille, France Bernstein Center for Computational Neuroscience - Göttingen, Germany http://www.nld.ds.mpg.de/~demian