Galaxy-galaxy strong lensing involves a foreground galaxy producing multiple distorted and highly magnified images of a background galaxy. Smaller perturbers, which can be either subhalos orbiting the foreground galaxy or halos along the line-of-sight, cause slight deviations in these bright arcs. For this reason, strong gravitational lenses have become a popular probe of structures at sub-galactic scales, which are crucial for discerning the properties of dark matter. For massive perturbers, we can distinguish between line-of-sight halos and subhalos by measuring the curl of the angular deflection field. Moreover, for lensing systems with bright source galaxies, we can measure the average power-law slope of the density profile of the perturbers, which can allow us to distinguish between different dark matter models. In the coming years, we expect to have tens of thousands of new strong lensing systems obtained by experiments such as the Vera C. Rubin Observatory, Dark Energy Survey, and others. Once these are followed up with higher resolution observations, measuring the abundance and density profile of sub-galactic structures with strong gravitational lensing will be a promising way to investigate the properties of dark matter.