Abstract
Knowledge of the coronary sinus (CS) anatomy is crucial for implantation of cardiac resynchronization therapy (CRT). Obstacles to CS entry, such as the Eustachian ridge and Thebesian valve, as well as within the CS, such as Vieussen's valve and the vein of Marshall, are important to understand and differentiate during implantation or to identify earlier by imaging. Anatomic knowledge is mandatory to select the most suitable side branch for lead implantation. Modern tools and techniques almost always enable other anatomic problems, such as tortuous, small, short, or overly straight side branches, to also be overcome.