NASA’s Twins Study consisted of a joint effort among 10 research teams across the country to characterize the physiological, molecular, and cognitive changes experienced by humans in response to exposure to spaceflight hazards. For the study, retired NASA astronaut Scott Kelly and his identical twin brother, retired NASA astronaut Mark Kelly, served as the two test subjects. Scott spent 340 days onboard the ISS while Mark remained on Earth. The results of the Twins Study reveal some interesting, surprising, and positively assuring data of how a single human body adapted to the extreme environment of space.