Simulations of galaxy evolution aim to capture our current understanding as well as to make predictions for testing by future experiments. Simulations and observations are often compared in an indirect fashion: physical quantities are estimated from the observational data and compared to models. However, many applications can benefit from a more direct approach, where the observing process is also simulated, so that the models are seen fully from the observer's perspective. To facilitate this, we have developed the Millennium Run Observatory (MRObs), a theoretical virtual observatory which uses virtual telescopes to `observe' semi-analytic galaxy formation simulations based on the suite of Millennium Run (MR) dark matter simulations. The MRObs produces data that can be processed and analyzed using the standard observational software packages developed for real observations. At present, we produce images in forty filters covering the rest-frame UV to infrared for two stellar population synthesis models, for three different models of absorption by the intergalactic medium, and in two cosmologies (WMAP1 and 7). Galaxy distributions for a large number of mock light-cones can be `observed' using models of major ground- and space-based telescopes. The data include lightcone catalogues linked to structural properties and model observation maps, as well as mock images and catalogues emulating the SDSS, CFHT-LS, DES, GOODS, ERS, CANDELS, UDF, and COSMOS surveys. Through the MR Web Portal, a publicly accessible tool facilitates exploration of the simulated images and access to many associated data products, including dark matter halo catalogues, semi-analytic galaxy catalogues, lightcone catalogues, and photometric catalogues from Source Extractor. (abstract taken from Overzier, Lemson et al. 2012)