The Interpersonal Reactivity Index (IRI) is a widely used self-report instrument designed to measure individual differences in empathy through a multidimensional framework. Comprising four distinct subscales—Fantasy, Perspective Taking, Empathic Concern, and Personal Distress—the IRI captures various aspects of empathic responsiveness. This chapter provides an in-depth review of the IRI’s psychometric properties, including its reliability and validity across different populations. We discuss the factor structure of the IRI, highlighting its consistent performance in measuring the specified constructs, while also addressing potential challenges such as cultural variability and item interpretation. Additionally, the chapter explores the use of the IRI in diverse research settings and its applicability in clinical and community samples. Limitations, such as potential biases in self-reporting and varying factor structures in different contexts, are considered. Despite these challenges, the IRI remains a robust and versatile tool for assessing empathy-related constructs.