RNA-binding proteins (RNA-BPs) play an essential role in key processes in all living organisms, from viruses to mammals. They are involved in RNA packaging, pre-mRNA splicing, translation, and RNA localization, to name a few. For these proteins to function properly it is imperative that they recognize their correct RNA target sequences or structures. As yet, relatively little is known about the mechanisms by which RNA-BPs specifically recognize RNA. To gain more insight into RNA-protein interaction, several genetic systems have been developed in the last few years (1 –4 ) (see Chapters 13 and 14 ). The system described in this chapter is based on in vitro genetics. This means that the selective step (in this case, the binding of an RNA-BP to RNA) occurs outside of a living organism, in the test tube.