Millennium Bridge was designed to carry large load and a degree of structure’s movement was expected and taken into account. However, after about 80 000 people crossed it on the opening day, some vibration was detected. The bridge began to sway sideways noticeably, and the movement became so strong that people could not walk steady any more; many had to cling to the sides of the bridge to maintain balance. The phenomenon of bridge sideways movement is not unique to the Millennium Bridge only. There were other structures, completely different from the given bridge that, to a various degree, suffered the same effect. However, those cases have not been widely publicized, thus the phenomenon, known as Synchronous Lateral Excitation, was not anticipated and has not been given enough attention by bridge engineers. When people walk they have a natural sway motion. This very motion causes small regular vibrations, which, as the result of chance correlation, generated slight lateral movement of the bridge. When that happened, pedestrians instinctively adjusted and synchronized their motion with bridge’s movement to counteract the effect and to walk more comfortably.