Domain-specific dataflow visual programming languages are now commonplace throughout the world of computing. Although not the earliest form of visual language (that honor, like so many others, of course is due to Sketchpad [27]) dataflow visual languages are now by far the most common form of visual programming language used in practice. Although they have been mostly unsuccessful as general-purpose programming languages, dataflow visual languages have excelled within specialised niche domains, including information visualisation [22], programmable logic controller programming [15,1], and business process re-engineering [14]. Even in just one domain, computer music, several dataflow visual languages have been successful as products over relatively long terms, including Max [7], Bars-n-Pipes [13], and the topic of this chapter, the visual programming language underlying the Nord Modular series of synthesizers [6].