This chapter analyses the process of the politicisation of transnational party co-operation in the European Parliament (EP), with special reference to the 1970s. It illustrates the uneven development of transnational party co-operation in the EP. The chapter deals with the two competitive characteristics of the politicisation of the party groups. An important background factor of the fourth historical phase of party co-operation in the European Community (EC) since 1969 has been the increased constitutional power and political influence of the Parliament. The organisational dominance was further underlined by the non inscrit members being by contrast under-represented in all the parliamentary bodies and receiving no secretarial facilities or offices until 1977. Similar specialisms have also been found in the Christian Democratic group, but its less intensive concern with specific policy formulation has muted national divergences. The Socialist group has always contained parties with different grades of interpretation, whether Socialist or Social Democratic.