In the endoscopic management of unresectable malignant biliary obstructions by placement of a metallic stent (MS), longer patency and a lower incidence of stent occlusion are desirable goals. With its mesh structure, the uncovered MS (UMS) is occluded mainly by tumor or tissue ingrowth, making it impossible to remove. The covered MS (CMS) was developed to overcome these disadvantages, and was shown to maintain patency longer than the UMS in our randomized study. The most important characteristic of the CMS is that it is removable, allowing it to be used in patients with resectable malignancies and benign strictures. In addition, the drug-eluting CMS provides an additional approach to the treatment of biliary malignancies. The CMS may also change the treatment paradigm for biliary strictures and strictures due to chronic pancreatitis. The CMS is analogous to a large-bore, expandable plastic stent and is effective both as an endoprosthesis and a dilating or anti-cancer device. However, to better understand the utility of these devices, we need to first consider mechanical properties such as radial force (RF, expansion force) and axial force (AF, straightening force). AF is particularly important when developing CMSs because of related complications.