A great deal of research has been ongoing in the medical informatics community in an attempt to create medical knowledge-based systems (KBSs) to assist the health care provider in both diagnostic and management decisions. In order for these systems to have a significant impact on health care, sharing of knowledge among institutions must take place. There are many challenges to sharing, including differences in vocabularies, local medical practice, local prior probabilities, syntax of the knowledge-base (KB), evaluation, royalties, and liability. The Arden Syntax has recently become an approved standard for representing knowledge with the purpose of allowing multiple users to create, criticise and share knowledge modules. While providing a common syntax for sharing by itself does not solve most of the challenges, it allows institutions to begin sharing, to see which issues are the most important and the most difficult. It has been found that differences in clinical vocabularies and data schemas provide the greatest current challenge to sharing practical KBSs. Thus, in order for KBSs to succeed on a large scale, ways to coordinate vocabularies and schemas across institutions must be developed. As sharing increases, evaluation and maintenance will become greater challenges.