The Indus Valley Civilization was a Bronze Age culture that developed between 3300-1300 BCE in what is now Pakistan and northwest India. Two major cities, Harappa and Mohenjo-Daro, had populations of up to 40,000 people and sophisticated urban infrastructure. The civilization declined around 1900 BCE due to unknown factors like climate change. Later, Indo-European speaking Aryan peoples migrated into the region, establishing Vedic culture by 1200 BCE and interacting with the remaining Indus Valley populations.