Abstract: Bacteria swim toward food (chemotaxis) by detecting temporal changes in their chemical environment and using this information to make a decision to swim straight or change direction. The chemotaxis system is remarkable for its sensitivity to multiple chemicals, over a wide range of concentrations. These properties rely on cooperativity within teams of receptors and on an adaptation system in which chemoreceptors are modified by methylation/demethylation. I'll discuss recent experiments that reveal new aspects of adaptation, including "assistance neighborhoods" in which tethered modifying enzymes can act on multiple receptors, and adaptive variation in the sizes of receptor teams which adjusts gain, all contributing to optimal chemotactic behavior.