... Data . ... In the coming era of LSST, PanSTARRS, WFIRST, Euclid, ALMA, ELTs, JWST, and other facilities, we want to know: What science can we learn by injecting more astrophysics into mock catalogs, and how can we use mock data to maximize the science output of real data? ... We stress that this workshop is as much for "observers" to discuss how they use mock data, and how real data inform what mock observations should include but don't, as it is for "theorists" to discuss how they mock the universe...