Paintings, mostly due to deteriorations, are sometimes repainted, concealing in underlayers important features, dates, names and other information. Conservators are facing dilemmas as to whether to preserve these interventions and retrieve valuable hidden information, but in the last decades the evolution of spectroscopic techniques has contributed to such uncertainties. The current brief review explains the intentional repaint and presents the techniques used around the world to visualize the underpainting layers, and how these techniques have developed from a simple X-Ray radiography and an infrared (IR) photography to mobile devices with great imaging capabilities. Case studies include Byzantine icons and oil paintings.