Abstract The World Bank Administrative Tribunal has begun its second quarter-century with a jurisprudential flowering of extraordinary proportions. Mr. Hansen’s study, which builds on his earlier 25-year retrospective, comprehensively surveys the Tribunal’s numerous doctrinal developments during this time. In this article, which is part one of two, Mr. Hansen revisits two of the four subjects explored in his retrospective: (i) the roles of the contract of employment, Bank rules, international treaties and national laws in the composition of the pactum established between a staff member and the Bank; and (ii) the development of binding custom from the practices of the Bank, other institutions and national governments. The third and fourth subjects, which deal with the Tribunal’s use of general legal principles and precedents drawn from international and domestic tribunals, shall be handled in the forthcoming second part of this study. Extensively footnoted, Mr. Hansen’s study is intended for both academics and practitioners specializing in international administrative law and comparative international jurisprudence.