Solar cells based on organic-inorganic metal halide perovskites show efficiencies close to highly-optimized silicon solar cells. However, ion migration causes current-voltage hysteresis and long-term degradation, which impedes large-scale commercial applications. I will show that transient ion-drift measurements can be a powerful tool to study the activation energy, concentration, and diffusion coefficient of mobile ions, when measured within the correct frequency range, and with suitable delay times. In methylammonium lead triiodide (MAPbI3) perovskite, we identified three migrating ion species which we attribute to the migration of iodide (I-) and methylammonium (MA+). The estimated activation energies for the migration of mobile ions in the tetragonal phase are 0.37 eV for I- and 0.95 eV for MA+ near grain boundaries. The latter changes to 0.28 eV near the tetragonal-to-cubic phase-transition temperature. In the cubic phase, we find an additional activation energy of 0.43 eV which we attribute to the migration of MA+ ions in the bulk. We find that the concentration of mobile MA+ ions is significantly higher than the one of mobile I- ions, and that the diffusion coefficient of mobile MA+ ions is three to four orders of magnitude lower than the one for I- ions. From the associated timescale, we conclude that MA+ ions are mainly responsible for the observed current-voltage hysteresis in solar cells at typical operating temperatures. I will also present first sights into the influence of processing on the mobility of ions, a quantification which could lead to a better understanding of ion migration and its role in degradation of perovskite solar cells.