Traditional classification systems for lymphoedema are of limited use for the diagnosis of specific forms of primary lymphoedema. The understanding of primary lymphoedema has been impeded by confusing terminology and a tendency to simply divide patients into three categories based on the age of onset: lymphoedema congenita manifests at or shortly after birth, lymphoedema praecox is apparent before the age of 35 years and lymphoedema tarda manifests thereafter. The clinical presentation in the spectrum of primary lymphoedema disorders is very variable; the phenotypes of primary lymphoedema conditions vary in the age of onset, site of the oedema, inheritance patterns, associated features and genetic causes. Different inheritance patterns are recognised and there are numerous associated anomalies. Some subgroups, such as Milroy disease and Lymphoedema distichiasis, are well characterised, but others are not. A new clinical classification for primary lymphoedema has been developed as a diagnostic algorithm. Its use is demonstrated on 333 probands referred to our lymphoedema clinic. Grouping patients by accurate phenotyping facilitates molecular investigations, understanding of inheritance patterns, and the natural history of different types of primary lymphoedema. Descriptions of the diagnostic categories, some of which have not been previously clearly defined as distinct clinical entities, are illustrated by clinical cases.