This book addresses the critical "why," "how," and "what" of humanity''s relationship with planetary health and sustainability, offering both theoretical and practical knowledge. This book is important in tackling global challenges such as climate change, biodiversity loss, and social inequality. The world is at a tipping point, and to navigate this crisis, we need a deeper understanding of the human-environment connection, sustainable development, and our ethical responsibilities. By offering a holistic approach that includes environmental justice, ecological economics, and intergenerational ethics, this book provides the necessary framework to foster long-term planetary well-being. The how lies in bridging the existing knowledge gap between theory and action. It does this by combining diverse philosophical perspectives—ranging from deep ecology to the capabilities approach—alongside practical, and actionable strategies for sustainability. These perspectives are interwoven with real-world applications emphasizing justice, equity, and sustainability. The book unpacks complex concepts like transformative learning and epistemological pluralism, ensuring the knowledge is accessible and actionable for various stakeholders. The book''s core topics are sustainability ethics, social and environmental justice, human flourishing, and the role of technology in sustainable development. It provides a solid grounding in these areas, integrating theoretical knowledge and practical case studies. This knowledge is essential for stakeholders in policymaking, education, activism, and academia concerned with humanity and planetary health. It provides the tools to address existential risks, foster global cooperation, and drive meaningful change. Drawing on my decades of experience in environmentalism and academia, this book offers new insights and guidance for creating a more equitable and sustainable future for all.