Cancer of the pancreas, gallbladder, or bile ducts is the most common cause of malignant obstruction of the biliary tree. Patients who have unresectable tumors have a dismal prognosis in terms of survival and quality of life. In these cases 5-year survival is less than 2% and palliation, such as the establishment of a biliary drainage, is the only treatment available. Two types of stents are routinely used: plastic stents (PS) and self-expandable metal stents (SEMS). The first generation SEMS are uncovered and recurrent obstruction, most frequently caused by tumor ingrowth through the metal mesh, is seen in 16-46%. Recently, covered SEMS have been introduced to prevent tumor ingrowth. Covered SEMS are associated with stent occlusion in 14% of patients. As can be expected, the most frequent cause of stent obstruction in these patients is sludge formation. Stent migration, and cholecystitis and pancreatitis caused by obstruction of the cystic duct and pancreatic duct, respectively, have been suggested to occur more frequently with covered SEMS. To date, however, one randomized trial and three comparative studies compared covered with uncovered SEMS, have found only a non statistically significant trend towards more frequent occurrence of these complications.