An overview of the technique of optical coherence tomography (OCT) is presented, and a spectral OCT instrument especially designed for art diagnostics is described. The applicability of OCT to the stratigraphy of oil paintings is discussed with emphasis on examination of the artist’s signature. For the first time, OCT tomograms of stained glass are presented and discussed. The utilisation of Spectral OCT in real-time monitoring of varnish ablation is discussed with examples of ablation, melting and evaporation, and exfoliation of the varnish layer provided, for the first time.