Gallbladder mucocele (GBM) formation was rarely reported prior to 20 years ago and has emerged as one of the most common, deadly, and poorly understood causes of gallbladder disease in dogs. Multiple and large retrospective studies inform many aspects of diagnosis, concurrent diseases, surgical treatment, complications, and prognosis of patients with GBM. Studies of pathogenesis are fewer but are beginning to disclose underlying mechanisms for GBM formation.