References to prior case law and other legal materials play a crucial role in justifying judicial decisions in many jurisdictions worldwide. This chapter provides an overview of research using quantitative methods to investigate citations in judicial opinions, offering a literature survey, a roadmap for gathering and analysing citation data, and a discussion of methodological challenges and opportunities for advancement in this field. In its literature review, the chapter distinguishes various streams of work, including macro-level studies that describe and compare citation patterns across different legal systems and micro-level analyses of judges’ citation decisions. It argues that micro-level studies are particularly well suited to produce results that inform our comparative understanding of judge behaviour. However, this work faces methodological challenges related to using traditional quantitative methods in the context of citation data, which have largely gone unnoticed in the legal literature. The chapter demonstrates these challenges through simulations and discusses ways to overcome them. The chapter also discusses the importance of methodological advancement and new research directions in the study of citations in legal opinions more generally. Beyond overcoming the previously mentioned methodological challenges, opportunities for improvement include conducting research that explicitly compares citation patterns across jurisdictions and developing theories that connect citation networks to relevant features of legal systems. Pursuing these opportunities can bolster the credibility of research investigating citation behaviour and contribute to a more nuanced understanding of the factors that influence the use of citations in legal systems across the globe and judicial behaviour more generally.