The connective tissues diseases are a group of related disorders, which are primarily autoimmune and triggered by environmental factors in susceptible individuals. They usually affect multiple organs and most of them can result in a shortened lifespan. Fortunately they are also very uncommon, and even rarer in later life. They are usually diagnosed on a clinical basis and, to help this, most have strict diagnostic criteria. However, a lot of patients do not fit well into one particular category, but overlap with several. Indeed a single individual could have features of or fulfil the criteria for all the diseases discussed in this review. A further group of patients are ‘undifferentiated’, often having mild disease and initially failing to fulfil any set of criteria.