The Great Recession of 2008 was primarily caused by the subprime mortgage crisis, which involved risky lending practices and a housing bubble that led to widespread defaults on mortgages. Deregulation in the financial sector allowed for increased risk-taking, culminating in the collapse of major financial institutions like Lehman Brothers. In response, reforms such as the Dodd-Frank Act and Basel III Accord were implemented to enhance regulatory oversight and reduce systemic risk.