For centuries, people viewed cities as apart from nature, the province solely of human artifice. However, it is increasingly clear that cities and urbanizing regions are deeply entangled with the ecosystems in which they are embedded, from the scale of microorganisms to the planetary reach of urban-generated pollution. This course examines the nature of cities--that is, the ecologies that shape them and their effect on those ecologies. We will survey the key concepts co-occurring in all ecosystems including population, community, metabolism, systems, feedback, climate, and geography. We will also examine major themes and processes shaping the relationship of cities to their environments, such as biodiversity, habitat conversion, climate change, heat island effect, pollution and other externalities. In this context, understanding how biological organisms interact with each other and their environment in an urbanizing world is crucial to understanding how the natural world works, how ecology in cities is different or similar to non-urban systems, and how to bring cities into closer alignment with goals of sustainability, resilience, and repair.