Baxter's Q-operator is the quantum counterpart of the Baecklund transformation from the theory of classical Hamiltonian integrable systems. The Q-operator, which depends on a complex parameter called spectral, shares common eigenfunctions with the Hamiltonians of a quantum integrable system and satisfies a differential or difference equation in the spectral parameter. Using the Calogero-Sutherland model as an example we shall discuss the construction of the Q-operator and comment on its application for the separation of variables.