Computing is increasingly intertwined with our physical world. From smart watches to connected cars, to the Internet of Things and 3D-printing, the trend towards combining digital and analogue materials in design is no longer an exception, but a hallmark for where interaction design is going in general. Computational processing increasingly involves physical materials, computing is increasingly manifested and expressed in physical form, and interaction with these new forms of computing is increasingly mediated via physical materials. Interaction Design is therefore increasingly a material concern. – Welcome to a book on the materiality of interaction, welcome to a book on material-centered interaction design! In this introduction to this book, “The Materiality of Interaction – Notes on the Materials of Interaction Design”, I describe the contemporary trend in interaction design towards material interactions, I describe how interaction design is increasingly about materials, and I propose “Material-centered interaction design” as a method for working with materials in interaction design projects.