Title: Stellar Tidal Streams as Cosmological Diagnostics Abstract: Within the hierarchical framework for galaxy formation, minor merging and tidal interactions are expected to shape all large galaxies to the present day. As a consequence, most seemingly normal disk galaxies should be surrounded by spatially extended stellar tidal features of low surface brightness. As part of a pilot survey for such interaction signatures, we have carried out ultra deep, wide field imaging of isolated spiral galaxies in the Local Volume, with data taken at small robotic telescopes that provide exquisite surface brightness sensitivity (around 28.5 mag/arcsec2). This initial observational effort has led to the discovery of previously undetected extensive stellar structures in the halos surrounding these galaxies, likely debris from tidally disrupted satellites. In addition, our team confirmed and clarified several enormous stellar over- densities previously reported in the literature, but never before interpreted as tidal streams. Even this pilot sample of galaxies exhibits strikingly diverse morphological characteristics of these extended stellar features: great circle-like features that resemble the Sagittarius stream surrounding the Milky Way, remote shells and giant clouds of presumed tidal debris far beyond the main stellar body, as well as jet-like features emerging from galactic disks. Together with presumed remains of already disrupted companions, our observations also captured surviving satellites caught in the act of tidal disruption. A qualitative comparison with available simulations set in a Lambda-CDM cosmology (that model the stellar halo as the result of satellite disruption evolution) shows that the extraordinary variety of stellar morphologies detected in this pilot survey matches that seen in those simulations. The common existence of these tidal features around normal disk galaxies and the morphological match to the simulations constitutes new evidence that these theoretical models also apply to a large number of other Milky Way-mass disk galaxies in the Local Volume.