Book excerpt: This book examines the nexus of cities and oceans and the interrelations between the Sustainable Development Goals (SDGs) 11 and 14, just after the first two critical years following the milestone year of hope in 2015. It advocates for actions both for sustainable cities, the largest interconnected and only human ecosystem, and for the global ocean that is the largest physical ecosystem. Cutting-edge concepts and actions are presented by and for cities and oceans, following the global engagements during the years 2015-2017. In the era of global geopolitics, cities offer major democratic spaces between the micro-regulations of the local communities and the governance of the global commons. The role of education, trust, and citizen empowerment cannot be stressed enough. This book offers an evidence-based, holistic and integrated view of key urban and ocean sustainability issues at the horizon of 2030 and of 2050. The chapters cover the most prominent issues at the heart of the matter, and highlight systemic multi-stakeholder eco-responses towards sustainability with economic, social, environmental dimensions, including political and cultural aspects. This book offers a full exploration of cities and seas with an emphasis on vigorous paradigm shifts, redesigning human systems, and reconciling them with nature. Building on robust evidence, and transformational cases, it provides structured advice for world leaders, stakeholders and scholars.