Part 5 of this Handbook begins with a general introduction to the concepts and definitions underlying the notion of the blue economy and continues with an exploration of challenges and controversies confronting the sustainable development of the marine environment. The tensions inherent in the idea that the marine environment can be both exploited for human use and protected for its intrinsic natural values, reflect the origins of the concept of sustainable development itself and these are explored in the chapters presented in this section. These cover an examination of the ethics and values underpinning the blue economy, the role that blue finance can play in advancing sustainable development, particularly for tourism and local communities dependent on the oceans for their wellbeing. The views of various stakeholders on the quality and legitimacy of the governance of a range of blue economy initiatives are also explored, as is the extent to which, philosophically speaking, ocean development can be understood as genuinely sustainable, whatever the rhetoric may be. A concluding chapter from practitioners of marine permaculture completes the perspectives of the contributing authors on the complexities and contradictions within humanity's use of the many and various resources of the world's oceans and in the wake of COVID-19.