The commonly used classifications for ptosis do not have a unifying concept, and this may contribute to confusion rather than clarification. Based on the mechanisms that cause ptosis, all cases can be classified into one or more of the following categories: (1) neurogenic; (2) myogenic; (3) aponeurotic; and (4) mechanical. While none of these mechanisms is original, this paper presents them together for the first time as an inclusive and helpful classification of ptosis. Before placing a patient in one or more of the four categories, a thorough history must be taken, an examination must be done, and appropriate laboratory tests must be performed. The thought processes then necessary to classify the ptosis should clarify not only the site of the defect but also the prognosis and range of procedures that may be used to correct the ptosis.