This chapter discusses why and how county lines drug-supply networks began to emerge in provincial towns across the United Kingdom. It sets out several pre-conditions (variables) which, once ripened, led to the creation of county lines drug-supply networks as they are now known. The chapter also details how street gangs have evolved, leading the social field of the urban street gang into a state of flux. Contributing to this flux are new affiliates with new aspirations to seek out and create competitive advantage by locating and exploiting dormant domestic drug markets in new locations. The chapter then outlines the push–pull factors which make a county line host location attractive. It establishes the Evolutionary Models of CL Networks, creating a typology of different UK county line models increasingly adopting business modes of professionalisation and sophistication.