Primary sclerosing cholangitis (PSC) is a disease of the bile ducts that causes inflammation and obliterative fibrosis of bile ducts inside and/or outside of the liver. This pathological process impedes the flow of bile to the intestines and can ultimately lead to cirrhosis of the liver, liver failure, and other complications, including but not limited to bile duct and liver cancer. The underlying cause of the inflammation remains unknown, but elements of autoimmunity and microbial dysbiosis are suggested by the fact that approximately 75% of those with PSC also have inflammatory bowel disease (IBD), most often ulcerative colitis. The most definitive treatment for PSC is liver transplantation.