Walking is the most important means of transport for humans. Only when our walking ability decreases due to aging or injury, we come to realize how important the ability to walk is in our lives. To understand how we can increase walking performance after aging or injury, we studied an important aspect of walking: balance control; as without control of our balance, we would fall down before even making the first step. In this thesis, we studied balance control during walking on a split-belt treadmill, a treadmill with two parallel belts, on which people walk faster with one leg than the other. Although this is a challenging task, healthy humans flexibly controlled their dynamic balance to stay upright during split-belt walking. This can be compared with real-life walking flexibility, where to avoid pedestrians, walk up curbs or text while walking, we constantly change our steps to control our balance, without even thinking about it. Furthermore, we found that when balance is assisted during gait training, by holding handrails, you learn less from this training. This suggests that learning to control your balance is an important element of learning how to walk. Finally, we found that people post-stroke who increase their forward balance with their paretic leg after a slip or trip might lose their side-ward balance due to decreased control of the paretic leg. Further research is necessary to find out whether we can decrease this problem in post-stroke rehabilitation.