In many, probably most, gene transfer experiments, the aim is to express the transgene or transgenes and produce some kind of gene product. We have already encountered some examples where this is not the case, e.g. the gene-targeting strategies discussed in Chapter 6, and we shall consider some more in Chapter 9 when we describe how gene transfer is used for insertional mutagenesis. However, it is fair to say that in most cases, the experimenter needs to make some provision to control the expression of the introduced genes, which means it is necessary to provide those genes as part of an expression construct containing the information required for transcription, RNA processing, translation and in many cases post-translational trafficking of the product. Constitutive expression is suitable for many experiments in cultured cells, whereas in transgenic animals it is often more appropriate to restrict transgene expression to particular cell types, tissues or stages of development. In both cells and animals, more precise control of transgene expression can be achieved through the use of inducible expression systems, in which the transgene or its product is brought under the control of a small chemical inducer. In this chapter we discuss the principles of gene expression control as applied in transgenic systems and look at the different regulatory elements that are available for expression construct design.