The protection of the Fourth Amendment requiring a reasonable suspicion of criminal activity, extends to all government seizures and detentions, including brief border investigatory stops, and whether the stop is justified will depend on the totality of the circumstances. In the context of border stops, the totality may include characteristics of the area, proximity to the border, usual patterns of traffic and time of day, previous alien or drug smuggling in the area, behavior of the driver, appearance or behavior of passengers, model and appearance of car, and officer experience. In this case, the following evidence justified the stop: the area is completely rural; it was about 10:30 at night; two hours earlier, a seismic device at the border had been activated; from the border, it takes two hours to get to the location of the detention; the area is a notorious smuggling site; the stopped vehicle was driving slowly.