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 lead to cirrhosis of the liver, liver failure, and other complications, including bile duct and liver cancer. The underlying cause of the inflammation remains unknown, but elements of autoimmunity and microbial dysbiosis have been described and are suggested by the observation that approximately 75% of individuals with PSC also have inflammatory bowel disease (IBD), most often ulcerative colitis. The most definitive treatment for PSC is liver transplantation, though only a fraction of individuals with PSC will ultimately require it.