The use of near field communication (NFC) has expanded as rapidly as Bluetooth or similar technologies and shows no signs of slowing down. It is used in many different systems such as contactless payment processing, movie posters, security access and passport identification. NFC enabled devices include cell phones, credit cards and key chains. With the spread of any new technology come security vulnerabilities that malicious users will try to exploit. NFC is particularly vulnerable to what is known as a relay attack. The relay attack is similar to the man-in-the-middle attack but the data need not be unencrypted to be vulnerable. The relay attack is currently undetectable and unstoppable. Many solutions have been proposed but no real-world solution has been found that does not require significant changes to the NFC protocol, or even the hardware. In this work we propose a technique that uses careful timing analysis of tag communication to identify a transaction as dangerous and thus set off an alert of the potential threat. This could be built into mobile devices and readers already deployed and provide a level of security to the market not currently available while maintaining the protocols set forth by the ISO. A proof of concept has been built and tested on custom hardware as well as on an Android Nexus 4 to detect and prevent the relay attack. In this thesis we give an overview of security issues in NFC communication, describe the relay attack in detail, present our timing based countermeasures and its implementation, and give results of our evaluation of timing based relay prevention.