The microscopic structure of the polar (000 (1) over bar) and (0001) surfaces of lithium niobate is investigated by atomic-resolution frequency modulation atomic force microscopy and first-principles calculations. It is found that the surface reconstructs at annealing temperatures sufficiently high to drive off external adsorbates. In particular a (root 7 x root 7)R19.1 degrees reconstruction is found for the (000 (1) over bar) surface. Density-functional theory calculations show that-apart from the (root 7 x root 7)-a series of adatom-induced surface reconstructions exist that lower the surface energy and reduce the surface charge.