The increasing role of private sector agencies in the provision of supply teachers across Britain's schools raises three inter-related issues that need to be addressed during the current process of re-assessing the medium and long-term viability of this policy option. First, there is a danger that educational concerns and equity of teaching provision across schools conflict with the private sector concern to maximize agency revenue. Secondly, the legal definition of the agency as 'employer' and supply teacher as 'self employed' leaves many unanswered questions regarding how the employment relationship is defined and managed in practice; ambiguities over whether the school or the agency bears the risk and the extent to which the supply teacher is free to bargain individual terms and conditions, raise problems regarding quality of supply teaching and equity of status of supply teachers compared to permanently employed teachers. Thirdly, increased use of the contract as a means of delivering teaching potentially conflicts with longstanding notions of what constitutes professional duties and obligations among teachers - factors that are difficult to write into a contract. This paper explores these issues drawing on 60 interviews with a mix of supply teachers, school teachers, and employees from two branches of a large teacher agency. It concludes that new models of providing temporary teaching cover are needed both to address problems of escalating costs and quality standards and to enshrine common notions of professionalism and equity of terms and conditions among all teachers.