Stressed out. Under pressure. Run down. We tend to think of stress as a bad thing—and it can be. Traumatic experiences in early childhood can destroy mental health later in life, and chronic stress has been linked to illnesses such as cancer and diabetes. But stress can be good too. It helps us navigate an unpredictable world and determine which new experiences are good, bad, or dangerous. It also primes us to respond to similar situations in the future. The key with stress is to strike a healthy balance—getting enough at the right moments, but not too much overall. If the prospect of following that advice makes you tense up, take a deep breath: Science can help.