Hartry Field has recently proposed a programme for philosophy of science which he describes as "nominalizing physical theory".¹ The programme consists in setting out a "nominalistic" formulation of physics, and then showing that the theory which results by the addition of mathematical postulates to this nominalistic formulation is a conservative extension of the original theory. The main purpose of the present paper is to discuss the various precise ways in which this claim can be formulated, whether they are true or false, and what the philosophical bearing of the different forms are.