Use of the "white spaces" in the UHF TV band has been identified as an important spectrum resource, providing new unlicensed spectrum access in frequency bands with excellent propagation characteristics. As a result, regulators around the world have been working on technical measures that will allow the use of this spectrum without interfering with incumbent licensed users. Defining these measures is complex and can have major implications on the amount of white space actually available. This article looks at a number of aspects of defining, using and regulating white space and shows which parameters are critical. It compares regulatory approaches put forward in the UK and US and demonstrates how the current US approach results in much restricted spectrum access. It also assesses key factors for the technologies using white space and shows how all these factors influence the applications most likely to succeed.