This chapter reviews the existing literature on two types of linguistic phenomena, both related to dialect contact: on the one hand, the dynamics of language diffusion and convergence which lead to processes of ‘informal standardization’; and on the other, the mix of varieties that are found in urban centres following the demographic growth of the latter (usually due to immigration from other regions or countries). In the first part of this work, the study of these two phenomena is reviewed in the different historical phases of the literature on sociolinguistics and dialect contact, and a comparison is made between the two types of varieties which, according to most authors, result from each phenomenon, and which are respectively termed ‘unofficial spoken standards’ and ‘new urban vernaculars’ in this chapter. In the second part, the same issues are examined in detail in the literature on Arabic sociolinguistics. Here, the importance of examining the interaction between the processes connected to the two types of varieties is underlined, in light of the fact that the different strands of studies have focused on just one of these two aspects of Arabic dialect contact while neglecting the other.