This research examines the utility of the concept of institutional racism in explaining racial disparities in stop and search practice in the UK and US. The concept of institutional racism was introduced in 1960s America. The concept was politically powerful in expanding existing understandings of racial inequalities which focused on individual prejudice and cultural pathology, to showing how racist discourses can become embedded in the structures of social formation. There were a number of analytical weaknesses inherent in the term at its conception. The concept has been utilized at various points of history in the US and UK. The 1999 Macpherson Report brought the concept of institutional racism back to popular usage in the UK, particularly in discussions around discrimination and policing. Macpherson took as evidence of the existence of institutional racism the continued disparities in stop and search use. The power to stop and search people in the street suspected of criminal activity has long been a feature of British and American policing. Research in both countries has continually shown that these powers are being disproportionately exercised against ethnic minorities. Thus this thesis explores whether the concept is useful in explaining disproportionate stop and search outcomes. The research is based on a study of police officers from two forces in the UK and two police departments in the US. It uses semi-structured interviews, observations and draws on official policy documents and statistics. The purpose of the research is to gain an understanding of the circumstances and decision-making by officers as they conduct stop and search and to understand the context in which these decisions take place. The findings reveal that discriminatory outcomes in stop and search are the product of not only the actions of individual officers but also national and local policies and practices. These policies and practices are devised and implemented by social actors. The disproportionate outcomes not only result from racism but also prejudice based on class and gender. The concept of institutional racism reifies individual institutions and obscures the role of social actors in institutions, who shape the policies and practices of an institution. Without an understanding of the contexts in which people draw on race ideas and what features of their social position allows them to assert these ideas into the policies and practices of an institution we are unable to apportion responsibility and build reform agendas. Thus institutional racism fails to explain the disparities in stop and search use in the UK and US.