Since the beginning of anti-government protests in March 2011, Syrian security forces have killed more than 4,000 protesters, injured many more, and arbitrarily arrested tens of thousands across the country, subjecting many of them to torture in detention. The security forces have committed these abuses as part of a widespread and systematic attack against the civilian population, which means they therefore constitute crimes against humanity. Based on interviews with more than 60 defectors from Syria's armed forces and intelligence agencies, this report focuses on the individual and command responsibility of Syrian military commanders and intelligence officials for these crimes. Human Rights Watch names commanders and officials who gave both direct and standing orders to use lethal force against the protestors as well as to unlawfully arrest, beat, and torture the detainees. The report concludes that senior military and civilian leadership, including President Bashar al-Assad and the heads of the intelligence agencies, bear command responsibility for violations committed by their subordinates to the extent that they knew or should have known of the abuses but failed to take action to stop them. Given Human Rights Watch's conclusion that crimes against humanity have been committed in Syria, the pervasive climate of impunity for security forces and pro-government militias, and the grave nature of many of their abuses, Human Rights Watch believes that the United Nations Security Council should refer the situation in Syria to the International Criminal Court (ICC).