This book investigates how deference is structured in individual cases, and whether the treatment of deference differs by regime or over time. Adjudicative decisions thus form the focus of this study. This focus, however, necessarily entails treating adjudicative decisions as a meaningful source of data. Legal (doctrinal) and social sciences (empirical) research methods provide two competing methodologies through which such a study might be conducted. Chapter 3 introduces the theory and methodology underpinning the empirical study reported in Part II. It presents a theory of international adjudication and explains how an empirical study of adjudicative reasoning can remain ‘internal’ to the discipline of law despite borrowing methodological techniques and theories from ‘external’ disciplines. It explores the relative strengths and weaknesses of empirical and doctrinal approaches, and examines how adjudicative decisions can remain a meaningful source of legal data when analysed using an empirical methodology drawn from non-legal disciplines. It further outlines the methods used to identify deference in the private property decisions of the selected courts and tribunals.