This chapter introduces the foundational concepts of sustainability and their relevance to tourism development. It traces the evolution of tourism from mass tourism to sustainability-oriented approaches in response to environmental and socio-cultural concerns. The chapter presents the conceptual framework of sustainable tourism, emphasizing the Triple Bottom Line of people, planet, and profit. It also introduces key theoretical perspectives, including the Norm Activation Model and Ecological Economics, to explain behavioral and systemic aspects of sustainability. The integration of sustainability into tourism planning and policy is examined. The chapter highlights the global significance of sustainable tourism in addressing climate change, biodiversity loss, and inequality, providing a foundation for understanding the multidimensional nature of sustainable tourism.