This study examines the impact of early childhood health on cognitive ability in young Peruvian children. It analyzes health data, as measured by height-for-age z-scores, at ages 1-2 and 4-5 from the Young Lives Survey to determine if health has different effects during critical and sensitive periods of development. The results show that overall health in the first five years does not causally impact cognitive ability. However, for children who experienced stunting at some point, there is evidence that investment in health may have diminishing returns once stunting is alleviated, indicating a sensitive period of development. Weak identification tests also show that the instruments used may not adequately identify the effect of health in this population.