This chapter seeks to explore issues involved with balancing public security with individual privacy. To illustrate these issues specific reference is made to an ongoing project part-funded by the European Commission (EC) under Framework Programme 7: SECUR-ED (https://www.secur-ed.eu">https://www.secur-ed.eu). This is a demonstration project aimed at supporting mass public transport security and entails security and surveillance exercises in complex travel interchanges in selected major European cities. Passengers in mass transport interchanges are notoriously vulnerable to terrorist attack (as evidenced by the bombings in subways in London, Paris and Madrid and the toxin attacks in Tokyo – see below). There is a need to find ways to protect the travelling public, but many of the underlying issues highlight problems associated with surveillance and security in a range of other complex settings.