Estimates suggest that there may be as many as a thousand ‘Earth-crossing’ asteroids (whose orbits bring them inside the Earth’s) that are 1 km or greater in size, and untold numbers of related objects smaller than this. In the long run their current orbits are not stable, so these bodies are likely to crash into the Sun or a planet, or else be scattered out of the Solar System, within a few million years.
Probably about five asteroids a kilometre or more across are likely to strike the Earth, on average, every million years. Undoubtedly objects this size have struck the Earth on many occasions. The largest impact for which we have direct evidence happened 65 million years ago, and was associated with a mass extinction event that wiped out the last of the dinosaurs and many other kinds of life on land and in the oceans. This impactor must have been either an Earth-crossing asteroid or a comet about ten kilometres across.
Events of this magnitude are fortunately very rare, but impact by any asteroid big enough to penetrate the atmosphere with unabated speed would have globally significant consequences. A stony object less than about 100 metres across that strikes the Earth's atmosphere is likely to become sheared apart into pieces small enough to be slowed down by aerodynamic drag that they would not hit the ground at crater-forming speed. For example a grapefruit-sized fragment of an asteroid (a meteorite) that struck a house in Auckland, New Zealand in June 2004 merely crashed through the roof and then bounced off the sofa.
Objects, or fragments of objects, bigger than 100 metres must be considered as potential major hazards. If one struck the sea the resulting 'tidal wave' (more properly described as a tsunami) would break across the surrounding coastlands to a considerable height. If the impact was on land, apart from the total destruction within the crater, the accompanying blast wave would devastate a wide surrounding area. The loss of life and economic dislocation caused by the immediate damage in either case would be enormous, and to make matters worse enough debris could be thrown into the atmosphere to block out sunlight for long enough to cause global famine.
Although there are no known examples of a human having been killed by an asteroid, it can be calculated that, averaged out over millions of years, you're as likey to die in an aircraft crash as to be killed by a falling meteorite, and perhaps thirty times more likely to die through accidental electrocution. These are not insignificant odds, and so efforts have begun (despite a lack of co-ordinated central funding) to detect and catalogue all the 1 kilometre diameter objects in near-Earth orbits to see if any of them is likely to strike the Earth in the foreseeable future, and to give as much warning as possible of the approach of smaller objects which cannot be detected until they get close. So far, the closest observed approach to Earth was by a 30 metre diameter object designated 2004 FH that came within 43,000 kilometres in March 2004. The closest predicted approaches for known bodies before the year 2050 are of a 600-metre diameter asteroid 2001 WN5, which will pass us at a range of 255,000 kilometres on 26 June 2036, and the 200 metre asteroid 2003 MK4 that will pass at only 210,00 kilometeres on 3 January 2023. Fortunately for us, these are both actually comfortable misses.