Whether or not a moped is street legal depends on the state in which the driver is using the moped. However, 150cc engine mopeds are legal in many states, and often legality issues are based on safety and noise requirements.
Mopeds are small motorcycles, though the term may also apply to scooters and other motorized devices. It is not uncommon for mopeds to be illegally tuned in order to make them faster or louder. Before users tune their mopeds and make modifications to the engine, it is important to take a look at your state laws to ensure planned modifications do not break them.