Although advances in drug-eluting stents (DES) have substantially reduced the risk of coronary in-stent restenosis (ISR) and the need for target lesion revascularisation (TLR), ISR persists. There are several treatment options for ISR (conventional balloon angioplasty, cutting or scoring balloons, drug-coated balloons, repeat DES implantation or bypass surgery). Coronary imaging is mandatory to perform PCI on ISR. Optimal coherence tomography (OCT) is an excellent option to guide PCI, but its role in ISR-PCI remains unclear. The INSIDE OCT Trial aims to compare the acute performance of PCI for ISR, either guided by OCT and angiography or by angiography alone.