Responsive materials cover a breadth of types and many application fields. The common feature in all cases is a nonlinear change in properties or behavior as a result of a stimulus. The material response can range from a simple change in conformation or ionization state, through to phase transitions, bulk aggregation, or complete dissolution. As a consequence, sensing and actuation are the most investigated functions of these materials. In this issue, we have chosen to focus on responsive materials as exemplified by externally switchable, environmentally activated, and reversibly or controllably triggered systems. The chemistries of these materials, their physical properties, functional behavior, and activity are all linked, so we have aimed to cover the many disciplines underlying responsive materials through articles featuring areas that already span disparate research topics. These areas include drug delivery, smart surfaces, and nanotube transducers. The responsive materials field is growing in excitement as well as activity, and we hope that readers will gain an insight into this fascinating branch of materials science through this MRS Bulletin issue.