The material composition and morphology of vulnerable atherosclerotic plaque components are considered to be more important determinants of acute coronary syndromes than the degree of stenosis. Rupture of a plaque causes thrombogenic material to contact the blood, resulting in a thrombus. Rupture-prone plaques contain an inflamed thin fibrous cap covering a large soft lipid pool. Mechanically, rupture occurs when plaques cannot withstand the internal stresses induced by the pulsating blood. These stresses concentrate within/around the cap/edge, since the lipid pool cannot bear much stress. During plaque development these stresses further increase when caps become thinner, lipid pools become larger, or the difference in stiffness (modulus) between the cap and the lipid pool increases. Intravascular ultrasound (IVUS) strain elastography/palpography and IVUS modulus elastography are imaging techniques that assess local plaque elasticity (strain and modulus) based on the principle that tissue deformation (strain) by a mechanical stress is a function of its elastic properties (modulus). Combined use of these techniques provides clinicians an all-in-one modality for detecting plaques, assessing their rupture proneness and imaging their elastic material composition. This chapter describes the terminology and pathophysiology of vulnerable plaques and discusses the techniques behind, the methods for and the validations of the elasticity imaging techniques.