Coronary angiography is routinely used to guide percutaneous coronary interventions (PCI) despite obvious limitations of this lumen based approach. Intravascular imaging including intravascular ultrasound (IVUS) and optical coherence tomography (OCT) represent two techniques that provide essential information on preprocedural lesion characteristics (i.e., lesion severity, landing zone, and plaque composition) and the result after stent implantation (i.e., stent expansion and eccentricity, strut apposition, lesion coverage, tissue protrusion, and dissections). A total of 11 randomized controlled trials investigated the effect of IVUS-guided PCI with mixed results (1-11). Of note, studies including patients with an increased complexity [i.e., chronic total occlusion (CTO) or lesion length >28 mm] demonstrated a consistent benefit of IVUS-guided PCI as compared with angiography, mainly driven by a reduction of repeat revascularization for restenosis (MACE at 1 year: CTO-IVUS, 2.6% vs. 7.1%, P=0.035; IVUS-XPL, 2.9% vs. 5.8%, P=0.007) (1,2). In addition, IVUS was instrumental in guiding left main stem PCI in the recent EXCEL and NOBLE left main trials in more than 70% of patients, although these studies were not designed to investigate effects of intracoronary-guided imaging (12,13).OCT has a high spatial resolutions of 10-20 µm, which is approximately 10 times greater as compared with IVUS. Due to a lower tissue penetration, OCT is limited in determining the plaque burden, vessel size based on the detection of the external elastic membrane (EEM) at the minimal lumen diameter, which is one of the parameters used for IVUSguided stent sizing (14). The majority of previous IVUS studies reportedly applied the multicenter ultrasound stenting in coronary (MUSIC) criteria (Table 1) (15) with the key criteria of an in-stent minimal lumen area ≥80% of the average reference lumen area or ≥90% of the lumen area of the reference segment with the lower lumen size along with symmetric stent expansion. Notwithstanding the increasing use of OCT for PCI-guidance, standard criteria for this light-based technology have not been established.To date, the use of OCT for guiding PCI has mainly been evaluated in smaller studies, using surrogate marker endpoints (16,17). In the recent DOCTORS trial, the use of OCT-guided PCI was associated with a small but significant difference in post-procedural FFR (0.94±0.04 vs. 0.92±0.05, P=0.005) (18). In the OCTACS study, OCTguidance resulted in a lower proportion of uncovered struts at 6 months (4.3% vs. 9.0%, P<0.01) (19).Based on the well-established and currently still widespread clinical use of IVUS-guided PCI and the available aforementioned evidence on its effectiveness, one of the key questions is whether OCT-guided PCI using a specific protocol is comparable to IVUS-guided PCI in terms of lesion expansion. ILUMIEN 3 (20) was designed to fill this gap of clinically relevant evidence.