The more that twin and other multiple pregnancies are investigated, the more it becomes mandatory that collaborative studies are set up in order to attain the critical number of cases needed to achieve meaningful and reliable results. The European Multiple Birth Study (EMBS) aims to study two aspects of twin and multiple pregnancy: 1) management of pregnancy and labour, with emphasis on the prevention of preterm delivery; 2) accurate determination of zygosity, a prerequisite for the proper use of the twin method in a variety of fields, eg, congenital malformations, genetics, clinical investigations, etc. The rationale, the methods and the organisation of the study are described and discussed.