Stars that tidal fields pull away from globular clusters orbiting in the halos of galaxies form long thin streams. Star streams are potentially useful for inferring the numbers and masses of globular clusters present at high redshift where the clusters may play an important role in ionizing the universe and producing some fraction of the binary black holes that merge today. The star streams themselves are sensitive to the sub-halos that CDM cosmology predicts should exist in our halo. Although the basic dynamics of streams and the gaps that sub-halos should create in the streams is simple, the complex merger history of globular clusters in the galaxy requires guidance from simulations to analyze. Upcoming Gaia, LSST and other kinematic data offer the prospect of creating a good dynamical test for the existence of CDM small scale structure.