- Various theories have been proposed for the decline of the Harappan civilization, including environmental factors like floods, changes in river courses, increased aridity, soil salinity, and internal factors like decline in trade or administrative collapse. - The "Aryan Invasion Theory," popularized by Mortimer Wheeler, suggested a violent end to the civilization by invading Aryans. - Modern archaeological research has found little evidence of large-scale violence or invasion across the entire Harappan area at the time of decline. Evidence points more towards a gradual decline and abandonment of many sites due to complex factors.