How close is the Dirichlet-to-Neumann map to the square root of the corresponding boundary Laplacian? This question has been actively investigated in recent years. Somewhat surprisingly, a lot of techniques involved can be traced back to a newly rediscovered manuscript of Lars Hörmander from the 1950s. We present Hörmander's approach and its applications, with an emphasis on eigenvalue estimates and spectral asymptotics. The talk is based on a joint work with Alexandre Girouard, Mikhail Karpukhin and Michael Levitin[-]