In the vast majority of marathons and road races, yes, you can wear a hydration vest. They are particularly popular in trail races and ultramarathons where aid stations are far apart. However, many "World Major" marathons (like London, Boston, or New York) have implemented stricter security rules in recent years. For example, some races permit vests with soft flasks in the front but prohibit back bladders or large backpacks for security reasons (to ensure nothing is hidden). Others might require the vest to be transparent or have a specific liter capacity (usually 2L or less). It is crucial to check the "Prohibited Items" section of your specific race’s website, as a "no-backpack" policy could lead to you being stopped at the starting corral. For most local races, however, vests are welcomed as they allow runners to skip the "chaos" of water stations and maintain their own preferred electrolyte mix throughout the course.