Star formation begins when the densest regions in molecular clouds collapse under their own weight forming so called “young stellar objects” (YSOs), i.e. protostars surrounded by a gas+dust disc (known as “protostellar” or “protoplanetary” disc), expected to be the birth place of planets in this newly formed proto-exo-solar systems. The advancements during the past decade in our observational capabilities revealed that 10% of the observed protostellar discs in YSOs present large (~10 –100 au) dust and gas depleted cavities surrounding their protostars, as well as a number of spectacular features such as spirals, shadows and other non-axisymmetric over-densities. Discs with cavities have been historically referred to as “transition discs” and are among the brightest and most studied YSOs in close star forming regions. The presence of a massive planet (several Jupiter masses) interacting with the disc has been shown to be an effective mechanism for forming dust/gas cavities and the observed structures. The possibility that binary stars, i.e. much more massive and luminous than giant planets, could lie unresolved in the centre of transition disc cavities has been little explored, mainly because of the argument: “if they were present they should be visible”. However, both theory and observations hint that such binaries could be much harder to be detected than previously thought, supporting the idea that some transition discs could be in fact “circumbinary”. Shedding light on this issue represents the heart of the “Observing Binaries in Transition-Discs” (ORBIT-D) project. ORBIT-D has the ambitious goal of providing an ultimate answer to the question “Which transition discs are circumbinary discs surrounding undetected binaries?”. Identifying circumbinary discs in the transition disc population is key for answering a number of open questions about the process of star and planet formation — widely acknowledged as major research priority by the scientific community.