Late Heavy Bombardment

The Late Heavy Bombardment (LHB) or lunar cataclysm happened about 4.1 to 3.8 billion years ago. During this time, a large number of impact craters formed on the Moon. By inference, the same thing happened on Earth, Mercury, Venus, and Mars as well.
The evidence for this event comes from the dating of lunar samples. This shows that most impact melt rocks formed at that time. Many hypotheses have been suggested to explain how asteroidal or cometary material got into the inner solar system, but no consensus yet exists. 
One popular theory is that the gas giant planets (or at least Jupiter) migrated in orbit at this time: see Grand tack hypothesis. This caused objects in the asteroid belt and/or Kuiper belt to be forced into orbits which reached the terrestrial planets. 
Some think that the apparent clustering of impact melt ages near this time is an artifact of sampling.
This topic is part of the formation and evolution of the Solar System.