Materials for biological modulation, sensing, and imaging. This issue of MRS Bulletin explores the design of materials that have proved capable of reporting biological activities in cells and identifies current challenges. The articles also focus on future developments of multifunctional materials for sensing applications and therapies. A variety of bioactive materials are being designed to sense and modulate diverse cellular activities and function, while being monitored by various imaging modalities. The cover image depicts nanoparticles devised as a vehicle to deliver signaling molecules to target cells via specific receptor-ligand binding. The molecules released from the materials activate cellular expression and secretion of proteins crucial for the development, regeneration, and pathogenesis of tissues and organs. See the technical theme that begins on page 12.