This chapter develops a set of conjectures to flesh out a robust concept of sustainability, coalescing around the notion of complex sustainability. We present complex sustainability as an assemblage of two main components: socioeconomic sustainability and deep sustainability. The question about socioeconomic sustainability revolves around four axes: the need to go beyond green capitalism (or decoupling sustainability from profitability), sustainable innovation in the new Artificial Intelligence (AI) urban regime (or framing innovation around sustainability goals), cohesion versus competitiveness (or finding ways to design equity policies with a local focus), and public value (or developing bottom-up market-shaping and value-co-creating eco-strategies). These four elements represent fundamental vectors or directions for urban sustainability that dispute existing strategic public policies and private priorities. We define and articulate deep sustainability around the following discussions: urban ecology and nature (as self-organizing complex systems), low entropy (a desired sustainability state would be one of minimum entropy production), materialities (contributing to the nonanthropocentric thrust of complex sustainability), urban experience (on the role of agency in shaping human understanding and inquiry through civic frictions), and transdisciplinary sustainability (the integrative analytical approach for complex sustainability). In summary, this chapter fleshes out the notion of complex sustainability, which includes two different yet complementary and interrelated responses (socioeconomic sustainability and deep sustainability) to the disruptive complexity generated by megaprojects.