LucID is a multimedia expert system designed specifically to help users make a correct identification of a biological specimen or to correctly diagnose a particular problem. The program comes in two parts, a builder used in the creation of keys, and a player that enables the user to identify specimens or to diagnose a problem using the key. A major feature of LucID is its ease of use. This and other features of LucID are described and a number of examples are provided of how LucID is being used for identification and diagnosis, and to access information relevant to the item that is keyed-out. The role LucID is playing in recent decision support and training systems is outlined. Future possibilities for LucID are explored, including the role it can play in distance education, in training users to make accurate observations, and in teaching the logical processes involved in creating keys