Bottlebrush polymers are flexible "backbone" chains densely grafted with flexible "side chains"that have interesting and useful combinations of properties,such as low viscosity despite molar masses up to 100 million g/mol.Local crowding of the side chains around the backbone can generate tension in the backbone resulting in partially extended backbone conformation.The degree to which conformation of the backbone is perturbed by the crowding of the side chains is the central question of bottlebrush physics.