The growth of 'unconventional' water resources as a new resource frontier has been much touted over the last two decades and is transforming society's relationship with water in diverse contexts. Desalination and wastewater reuse, in particular, are increasingly framed together as potentially game-changing technologies for water management and (re)distribution and are carried forward by promises to overcome water scarcity and enhance water security. While there are good reasons to critique the conflation of heterogeneous water resources under the single heading of 'unconventional', we argue that the scale and scope of the transition towards desalination and treated wastewater (which often use similar technologies) merit their inclusion in one Special Issue. The papers presented in this issue advance our understanding of the social, political, economic and cultural dimensions of this water transition. The papers are conceptually and empirically diverse, with case studies across the Global North and Global South. They offer an important counterbalance to the dominant techno-triumphalist narratives that typically surround these technologies, providing unconventional perspectives on unconventional water. In this opening paper, we chart the emergence of unconventional water. We then introduce the papers and highlight the cross-cutting themes of the issue: 1) the (de)politicising discourses that frame desalination and wastewater; 2) the political economies of unconventional water; 3) the materiality and politics of these technologies; and 4) their implications for water justice.