Galaxy formation models, particularly the popular models based on Cold Dark Matter (CDM), have focussed their attention on galaxy halos, since these are overwhelmingly the most important components of galaxies in terms of their mass. Observationally, however, very little is known about these dark regions of galaxies, and even less is known about the ``outer halo'' regions, where structures can survive the disruptive Galactic tidal forces for more than a Hubble time. The advent of powerful wide-field cameras and wide-field multi-object spectrographs, now brings the study of these low-density outer regions of galaxy halos within reach. If CDM theory is correct, there should be about a hundred halo substructures in this volume. I will review our several searches for substructures in the halos of the Milky Way and Andromeda galaxies, the two best targets for this study, and discuss how the detected structures can be used to illuminate the dark and immensely massive region they inhabit.