Gravitational lensing of the cosmic microwave background (CMB) encodes information from the low-redshift universe. Therefore, its measurement is useful for constraining cosmological parameters that describe structure formation, e.g. Omega_m, sigma_8, and the sum of neutrino masses. In this talk, I will present a measurement of and the cosmological constraints from the CMB lensing potential and its power spectrum using data from the SPTpol 500 deg^2 survey. From the minimum variance combination of the lensing estimators from all combinations of SPTpol temperature and polarization data, we measure the lensing amplitude A_MV = 0.944 \pm 0.058 (Stat.) \pm 0.025 (Sys.), which constitutes the tightest lensing amplitude measurement using ground-based CMB data alone. Restricting to only polarization data, we measure the lensing amplitude A_Pol = 0.906 \pm 0.090 (Stat.) \pm 0.040 (Sys.), which is more constraining then our measurement using only temperature data. As SPT-3G, the successor to SPTpol, and other CMB experiments continue to lower the CMB map noise levels, polarization data will dominate the signal-to-noise of lensing measurements for angular multipoles below at least several hundred. Looking to the future, high signal-to-noise measurements of lensing enabled by deep polarization maps is crucial for constraining the sum of neutrino masses and the amplitude of inflationary gravitational waves through delensing. If time permits, I will give an update on the current effort of delensing the BICEP/Keck telescope data.