Abstract: Characterizing the ubiquitous dark matter in the universe has proven to be one of the most challenging problems in modern astrophysics. If the dynamical impact of dark-matter dominated dwarf galaxies on the outskirts of galactic disks can be deciphered, we may be able to infer and characterize cold dark matter sub-structure in a fundamentally new way. I show how one can analyze observed disturbances in the outer gas disks of spiral galaxies to quantitatively characterize galactic companions without requiring knowledge of their optical light. This method, which I call "Tidal Analysis", allows one to determine the mass and relative position (in radius and azimuth) of galactic companions from analysis of observed disturbances in gas disks. I will first demonstrate the validity of this method by applying it to local spirals with known optical companions to provide a proof of principle. I will then review my earlier work on the Milky Way that prompted the development of this method. Specifically, analysis of observed disturbances on the outskirts of the Milky Way disk favor a 1:100 mass ratio perturber with a close pericentric approach. I will conclude by discussing ongoing work on developing scaling relations between observed HI maps and satellite mass, and our plans for the near-future which include testing the Tidal Analysis method on large samples to determine its statistical viability.