How is legal expertise deployed in an institution such as the Westminster Parliament? The sociology of the legal profession suggests that the client plays a major role in how legal expertise is understood and used. In the case of Westminster, this is somewhat problematic because Parliament as a ‘client’ is not easily pinned down: it is multicentred-institution and parliamentarians; and it is an intensely political arena. The result is that there is a wide variety of sources of legal expertise (this chapter prefers the term ‘legal support’) for Parliament and parliamentarians, and no one source has a monopoly over legal support, save where the issue relates to institutional matters. The ultimate argument of the chapter is that the processes of seeking and making use of legal support is not that different from the way that legal support is sought and made use of in society more broadly. The client determines when they will seek legal support, and what use they make of it. In practice, then, parliamentarians make use of known sources first to understand legal issues (which may come from inside or outside Parliament); only rarely will they seek or need to seek formal ‘legal advice’.