The Aid Worker Security Report 2011 is the third in a series of briefing papers monitoring trends and issues in security for humanitarian operations that base findings on data from the Aid Worker Security Database (AWSD). The AWSD tracks reports of major incidents of violence against aid workers worldwide, including killings, kidnappings and armed attacks that result in serious injury. Aid workers are defined as the personnel of UN humanitarian agencies, Red Cross/Red Crescent movement, and NGOs with programmes in humanitarian contexts (see a detailed methodology description at the end of this paper). The authors originated the AWSD and analytical methodology in 2004, under a collaborative project of the Humanitarian Policy Group (HPG) and the Center on International Cooperation (CIC) called Providing Aid in Insecure Environments. The AWSD is now a project of Humanitarian Outcomes, an independent research group that maintains collaborative affiliations with HPG and CIC. In 2010, an online version of the database was launched to allow free public access to the data, made possible by grants from the governments of Canada, Ireland, and the United States. In addition to presenting an updated analysis of statistical trends, the Aid Worker Security Report 2011 highlights the issue of security for national aid workers, drawing upon findings from a major survey of national aid workers conducted for a recent OCHA-commissioned study: To Stay and Deliver: Good Practice for Humanitarians in Complex Security Environments (Egeland, Harmer and Stoddard 2011).