This book examines the role of data within interaction design, with a particular focus on the practice of data physicalization, an approach that aims to make data tangible and sensorially experienced. Moving away from the idea of data as abstract or neutral, this book presents it instead as situated, embodied, and shaped by context. Through a combination of literature review and case studies, it explores how data can be materialized and engaged with through physical artefacts and interactive systems. Drawing on insights from design research, human-data interaction, and science and technology studies, it outlines key principles and introduces two preliminary tools to support the design of data physicalizations: a variables scheme and an ideation process. These tools are conceived as resources to be refined and tested through iterative, practice-based research. The final chapter highlights three emerging directions that call for further exploration: mapping data through living and responsive materials, adopting a more selective and thoughtful relationship with data, and using physicalization to support more autonomous and reflective forms of data engagement. Overall, this book aims to offer both a conceptual grounding and practical entry points for designers and researchers interested in approaching data as a material to think with, and design for.