Percutaneous coronary intervention has been developed for patients with coronary artery disease. Calcified lesions are recognized as an unsolved issue where many clinical devices have evolved and some disappeared. Understanding intracoronary imaging of the calcified lesions can help operators to make decisions during the procedure. There are several potential stories of progression of calcification, although a precise mechanism of progression of calcification remains unknown. In the process of a large calcification, it is histologically believed that lipid is replaced by calcification. This process can be observed by intracoronary imaging devices, i.e., intravascular ultrasound and optical coherence tomography. Calcified nodule is a unique type of calcifications. Among the calcified lesions, especially calcified nodule has serious clinical outcomes such as target lesion revascularization (TLR) with stent under-expansion. Additionally, in-stent calcified nodule is a distinctive type of restenosis pattern after stenting to calcified nodule, leading to malignant cycle of repeated TLR. Recently, calcified nodule is divided into two types based on the surface irregularity: (1) eruptive and (2) non-eruptive calcified nodule. Eruptive calcified nodule has higher rate of target vessel revascularization than non-eruptive calcified nodule despite greater stent expansion in eruptive calcified nodule. It is thought that there are differences of component such as the amount of fibrin and the size of calcific nodules between both, although it is common for both to include calcific nodules and fibrin. Histopathological understanding calcified nodule can be helpful to choose the treatment devices during the procedure in the area where there is no correct answer.