This thesis assesses the legal effect of Community Agreements, explored through the case-law of the Community courts. It places this issue within the broader setting of the legal effect of treaties in domestic legal orders and how we think about the role of domestic courts in treaty enforcement. It proposes a basic dichotomy between automatic and non-automatic treaty incorporation in preference to the commonly employed, but analytically unhelpful, language of monism and dualism. And it emphasises the need for greater empirical work as to how courts in automatic treaty incorporation states actually deal with treaties when they are invoked; rather than relying on the untested assumption that the particular phrasing of a constitutional provision providing the port of entry for treaties into the domestic legal arena and/or seminal judicial assertions on their legal effect is matched by existing judicial practice. To this end, a data-set of the existing Community Agreements jurisprudence of the Community Courts was created. It is an assessment of this data-set that provides the core empirical work of this study. This study illustrates how the foundational Community Agreements jurisprudence signalled an attachment to an automatic treaty incorporation model and thus erected a central plank of the Community's external relations constitution with profound constitutional ramifications for the Member States. This constitutes a neglected dimension of the constitutionalisation debate, namely, the constitutionalising effect of Community law upon Community Agreements. The data however indicates that there is evidence of a twin-track judicial approach to Community Agreements emerging. The first, where domestic action of the Member States is being challenged, appears to manifest the unleashing of a full treaty enforcement model. In contrast, there are indications of a judicial willingness to shield Community action from review vis-àvis Community Agreements which has significant implications for the EU's commitment to international law.