This chapter deals primarily with the evidence available regarding the use of the term unclassified or undifferentiated connective tissue disorders (CTDs) to refer to patients with manifestations suggestive, but not diagnostic, of either defined CTDs, a well-defined overlap syndrome (manifestations and criteria for two or more CTDs), or mixed CTD (MCTD). Possible outcomes for these patients include remaining undifferentiated, differentiating into a defined CTD or going into remission. MCTD and atypical CTD (term used by the parties involved in the 'Silicone breast implant' controversy) are only briefly discussed.