There is an increasing body of evidence supporting that both fitness and fatness levels relate to current and future individuals’ health status. In this article, we discuss the meaning of fitness and fatness/obesity, and make an overview of what is currently known about fitness and fatness as potentially modifiable risk factors related to health and disease from preschool children to older adults. We describe the methods available for fitness assessment in each age group, providing reference/criterion values when available. Most of the existing previous reviews are focused on specific age groups with the advantage of allowing more in-depth analysis of the evidence, but the disadvantage of losing the overall understanding of the fitness and fatness binomial through the human lifespan, which is the ultimate goal of the present article.