There are two theories on the birth of the Harappan Civilization, one that was completely indigenous and the other that it was a result of influence from Mesopotamia. It is generally believed that the origins of Harappa were largely indigenous but that it had significant trading links with Mesopotamia. Some historians, such as Shireen Rutnagar have also suggested that the decline of Harappa was due to a decline in these trade links. Ancient Mesopotamian texts mention links with Meluhha which is believed to be a reference to the Harappan civilization. The civilization is now divided into four main phases based on the level of urbanization prevalent.

The early excavations at the sites were carried out in the early 19th century by the explorers Charles Masson and Alexander Burnes, and the archaeologist Sir Alexander Cunningham in the 1870's. This work led to the the first excavations in the early 20th century at Harappa by Rai Bahadur Daya Ram Sahni, and by R.D. Banerji at another Indus Valley city, Mohenjo-daro. It was discovered that some of the ancient bricks of the civilization had been used to build the Lahore railway!

It must also be mentioned that the use of the term Indus Valley Civilization to describe the Harappan Civilization is now believed to be largely inaccurate as the civilization extends from distant parts of Afghanistan to modern day Bihar in India. Newer sites have been discovered in recent years. Basic questions about the people responsible for creating this highly complex culture remain unanswered.

An interesting feature of the civilization was the level of town planning that was attained, with precisely built roads, drains,excellent sewage and sanitating facilities, the Great Bath at Mohenjo Daro and huge granaries. The Harappans used the same sized weights and standardised weights for thousands of miles, a feature that characterises the civilization.

The two key sites of the civilization are Harappa and Mohenjo-Daro while the key port in present day India is Lothal. Excavations at Harappa have yielded five mounds and a large structure possibly used as a granary. Mohenjo-Daro is the the biggest site and is located in present day Sindh, Pakistan. Other major sites include Rakhigarhi, Dholavira, Suktagen-Dor, Chanhudaro and Ganeriwala.

There are references in the Rig Veda to the destruction of Harupiyah by Indra and there have been suggestions that Harupiyah refers to Harappa. However, these claims remain largely unsubstantiated and there was a time gap of a 1000 years between the compilation of the Rig Veda and the end of the Harappan Civilization.

The decline of Harappa too is a matter of controversy. It is now generally believed that there is little evidence to support Mortimer Wheeler's Aryan invasion and massacre theory. There were theories by Raikes and Dale about a possible flood but none of these views have been substantiated. There are also theories that the Saraswati or the Ghaggar Hakra river gradually dried up and strangled the civilization of its resources. It is now widely believed that in Harappa and Mohenjo-Daro there was a gradual process of de-urbanization aided by a number of factors- floods, frequent invasions by hill tribes etc and this resulted in a mass movement of people. The Harappan culture then declined to the Cemetery H culture which was characterised by a far inferior brand of cultural artefacts.

Finally, the Harappan script has been the source of much controversy. The script, found mainly on seals that were possibly used for commercial purposes, is yet to be deciphered and there are suggestions by many right wing historians, not taken seriously by most academicians, that the script has Aryan sources. But more recently, the Finnish historian Asko Parpola has suggested that instead of Sanskrit, the source of could have had Dravidian origins. There is no conclusive evidence on either side.