Thin, dynamically cold stellar streams such as GD-1 and the Palomar 5 stream trace almost great-circle orbits in the Galactic halo, with widths of only a few tens of parsecs and very low internal velocity dispersion, making them exquisitely sensitive to the underlying Milky Way potential and to perturbations by dark subhalos (Odenkirchen et al. 2003; Grillmair & Dionatos 2006). In ΛCDM, a rich population of dark matter subhalos should repeatedly disturb such fragile streams, creating noticeable gaps, kinks, and thickening, yet observations of GD-1, Pal 5, and other halo streams constrain the number and mass spectrum of substructures and sometimes appear in tension with the abundance and impact of subhalos predicted by standard cold dark matter simulations (Yoon, Johnston & Hogg 2011; Bovy et al. 2017).