Over the past two decades, there has been an increasing incidence of biliary mucocoeles in dogs. This increase cannot be attributed simply to the increased use of ultrasound or breed predispositions. The development of mucucoeles is complex and associated with changes in mucin composition, biliary composition and likely genetics. Most mucocoeles require surgery in a timely manner. However, there are a variety of changes in the gall bladder appearance that precede the development of mucocoeles, and decisions need to be made about how and when to intervene in these cases.